In This Chapter
^ Switching from fractions to decimals to percents and back again ^ Investigating both the practical and impractical with percents ^ Using percentages to your advantage — in your best interest
A Decimals and percents are really just fractions — in a more manageable
Format. Doing problems that involve percents of things involves changing the percents to decimals and then doing the indicated operations. That’s not a big deal, if you handle the decimals correctly. And the computations are much easier than with fractions, which can have very uncooperative denominators.
In this chapter, you see how to figure percent increase and percent decrease and determine whether what you see advertised is a good deal. Everyone is affected by interest on money — whether you’re borrowing or saving — so I include problems dealing with computing interest as well.
Relating Fractions, Decimals, and Percents
The usual move from fractions to percents is through decimals — the decimal format is the middleman in the process. You probably already know some of the more common equivalences of percents and fractions. You know that 50 percent is equivalent to >2 and 25 percent is equivalent to!4. Well, I’m assuming that you know this, but, just in case, here are some properties and techniques that you can use to make the transitions easier. You don’t have to memorize these properties, but having some of them in mind as you’re working on percentage problems is helpful.
Changing from fractions to decimals to percents
A decimal is a fraction and vice versa. To change a fraction to a percent, you first determine the decimal value and then fiddle with the decimal point. That’s all there’s to it — really.
To write a fraction as a decimal, you divide the numerator by the denominator, inserting the decimal point where needed. To change a decimal to a percent, you move the decimal point two places to the right and use a percent sign (%).
For instance, changing the fraction j-^j to a percent, you first find the decimal
By dividing the numerator, 7, by the denominator, 16. The decimal that you get is 0.4375. Here’s what the division looks like:
0.4375
16 )7.0000
64 60 48 120 112 80
80
To change the decimal to a percent, you move the decimal point two places to the right to get 43.75 percent. This makes sense, because 7 is not quite half of 16, and 43.75 percent is just short of half of 100 percent.
Finding terminating decimal values
Fractions all have decimal values, but some of these decimals Terminate (come to an end) and some Repeat (never end). As long as the denominator of the fraction is the product of 2s and 5s and nothing else, then the decimal equivalent of the fraction will terminate. To find this terminating decimal, you divide the denominator (bottom) of the fraction into the numerator (top) and keep dividing until there’s no remainder. You may have to keep adding 0s in the divisor for a while, but the division will end.
For example, to find the decimal equivalent for the fraction -25, you divide 25
Into 14. To begin the division, you put a decimal point to the right of the 4 and add a 0 to the right of the decimal point. You keep adding 0s as needed in the division. This is what I get when I do the division.
0.56 25 YT41)0 125 150 150 0
So 25 = 0.56. As you see, this decimal Terminates. The division stopped
Because I eventually ended up with no remainder. You can predict that this will happen (the termination of the decimal), because the denominator, the 25, is equal to 5 times 5. The only factors of 25 are 5s.
Next, you see the division required to find the decimal equivalent of the
Fraction 119g. This decimal will terminate, too, because the denominator is
Equal to 2 X 2 X 2 X 2 X 2 X 5. Only 2s and 5s are factors of the denominator. It doesn’t matter what the numerator is; the decimal will terminate. Finding the decimal equivalent:
0.050625 160 G 9.000000 800 100
_0
1000 960 400 320 800 800 0
It took a while, but you can now say that -MTt = 0.050625.
160
Computing decimals that keep repeating themselves
Terminating decimals are just dandy, but they’re in no way the only type of decimal value out there. Repeating decimals occur when you change a fraction to a decimal and the denominator of the fraction has some factor other than 2 or 5. It only takes one such factor to create the repeating situation. For
Example, the fraction 12 repeats when you divide the numerator by the denominator, because the denominator has a factor of 3.
0.4166… 12 G 5.0000
48 20
12
80
72
80
72
8
As you can see, the remainder will now forever be 8, and the corresponding number in the quotient (answer of a division problem) will be 6. The three dots following the last 6 shown indicates that the 6 keeps repeating forever and ever.
To indicate a repeating decimal, you can either write three dots (ellipsis) after some repeated digits, or you can draw a horizontal bar across the digits that repeat. For instance, 0.41666 . . . can also be written 0.416. Note that the bar is over the 6 only. The first two digits don’t repeat.
When you’re using repeating decimals in a problem, you decide how many decimal places you want and then round the number to that Approximate Value. Rounded decimals aren’t exactly the same value as the repeating decimals, but you can make them pretty accurate in an application by using enough digits in the decimal equivalent.
Making the switch from fractions to percents
The middle step in changing a fraction to a percent is finding the decimal equivalent (or, in the case of a repeating decimal, the approximate). Table 6-1 shows you some fractions, their decimal value, and then the percent that you get by moving the decimal point two places to the right.
|
Table 6-1
|
Fraction, Decimal, and Percent Equivalences
|
|
Fraction
|
Decimal Percent
|
|
3 4
|
0.75 75
|
|
9 16
|
0.5625 56.25
|
|
1
3,125
|
0.00032 0.032
|
|
Fraction
|
Decimal
|
Percent
|
|
9
160
|
0.050625
|
5.0625
|
|
1
3
|
0.333 . . .
|
(0.3333 when rounded to 4 places)
|
33.33
|
|
47
66
|
0.71212 .
|
. . (0.7121 when rounded to 4 places)
|
71.21
|
|
5 12
|
0.41666 .
|
. . (0.4167 when rounded to 4 places)
|
41.67
|
|
1
999
|
0.001001
|
. . . (0.0010 when rounded to 4 places)
|
0.1
|
Changing from percents back to fractions
Percents are very descriptive — they tell you how many you have out of 100 things. The only problem with percents is that you can’t use the percent format when doing computations. You need to change a percent back to a decimal or a fraction, if you want to multiply or divide using the percent.
To change a percent to a decimal number, move the decimal place in the percent two places to the left, adding 0s if necessary. To change that decimal number to a fraction, write the digits in the decimal over a power of 10 that has as many zeros in it as there are digits in the decimal — then reduce the fraction if you can.
For example, 45 percent has a decimal value of 0.45, and the fraction equiva -
Lent is
45 _ 9
. Another example is 0.032 percent, which has a decimal 32 1
Value of 0.00032 and a fraction of, AA AAA = 010,.
100,000 3125
Tackling Basic Percentage Problems
The nice thing about percents is that their values are easy to relate to. If you’re 75 percent finished with a project, then you know that you’re well on your way. You compare the percentage to 100 — a nice, round number — and have a good idea of what the value is in the comparison. To be more exact with an answer, though, you need to convert percentages to decimals and create a more exact value to use in computations. Using the decimal equivalents, you can solve for the percent of a value and get the answer in items, and you can also solve for how many items are needed to reach a certain percent.
Finding the percent amount
When you’re told that you have 60 percent of the work done or 85 percent of the problems correct, you multiply the total number of hours needed to do the work or the total number of problems on the test by the percent to get the numerical value of what you’re discussing. Percents are convenient amounts for comparison. You convert percents to decimals to use them in problems.
The Problem: You sign up for Weight Watchers, and you’re told that you need to lose 10 percent of your current weight. If you weigh 160 pounds, how much do you need to lose?
Changing 10 percent to a decimal, you get 0.10. Multiply 160 X 0.10 and you get 16 pounds. That should be a piece of cake. Oops! Not on Weight Watchers — make that a carrot stick.
The Problem: You’re told that 95 out of 100 of the people who buy a Honda motorcycle will buy another Honda when they need to buy another motorcycle. Last year, the number of Honda motorcycles sold in North America was 570,000. How many of these owners will buy a Honda when they make their next motorcycle purchase?

^VLA/jf First, change the fraction to a decimal. The value of 95 out of 100 is 95
Percent, but you can’t use the percentage in a computation — you want the decimal value. This fraction is equal to 0.95, so 570,000 X 0.95 = 541,500 people who will buy a Honda motorcycle the next time they make a purchase.
The Problem: To get an A in your math class, you need to have an average of 92 percent of all the points available. You currently have 540 out of a possible 600 points, and you still have the 200-point final to take. What do you need to score on the final to get an A?
^VLA/V This problem involves several operations: addition, multiplication, and subtraction. First, determine the total number of points needed. Add 600 + 200 for a total of 800 points. You need 92 percent of 800 points to get an A, so multiply 0.92 X 800 = 736 points. You currently have 540 points. Subtract 736 – 540 = 196. You need a score of 196 out of 200 points, which
Is 196 = Tkr = 0.98 = 98 percent. You have your work cut out for you. 200 100
Finding the whole when given the percent
Working backward to find out where you get a certain percent amount requires division instead of multiplication. This process of using division makes sense, because multiplication and division are inverse operations — one Undoes The other.
The Problem: Forty-five percent of the class is boys. And you’re told that the class has 18 boys in it. How many students are in the class?
Falling golf balls
Two large barrels contain the same amount of height above the two barrels, are dropped at water. The water temperature in the first barrel exactly the same time. Which ball will touch the is 49°F, and the temperature in the other is 29°F. bottom of the barrel first? Two identical golf balls, positioned at the same
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VLAiV Divide 18 by 45 percent — 18 0.45 = 40. There are 40 students in the class. You don’t believe this works so easily? Then check the work by finding 45 percent of 40 to see if you get 18 boys. Multiplying 0.45 x 40 = 18 boys. By golly, it works!

The Problem: In a large bag of Skittles, 20 percent of the candy is colored yellow and 18 percent is colored red. If you counted 100 yellow candies, then how many are red?
^VIA* First, determine how many pieces of candy there are altogether, and then determine what 18 percent of that number is to find out how many are red. You know that 20 percent of the candy corresponds to 100 pieces, so divide 100 by 20 percent — 100 ■ 0.20 = 500 pieces of candy. Now multiply 500 times 18 percent, or 500 x 0.18 = 90 pieces of candy that are red in color.
Looking At Percent Increase and Percent Decrease
You’ve been drawn to a store when it advertises "All Prices Slashed 20 Percent" or "Take 15 Percent Off the Reduced Price." Who wouldn’t be tempted when you’re offered such deals? And what about that meeting with the boss when she says that you need to increase productivity by 25 percent? Where does that put you as far as output? Can you do it? Percent decrease and percent increase are both based on changing the amount from 100 percent, or the full amount. You use the difference from 100 percent to help you when doing the problems.
Decreasing by percents
Figuring a percent decrease — or the new value of an item after the decrease is applied — just takes a deep breath and a little common sense. You can get messed up with the arithmetic if you don’t think about what the answer should be ahead of time. In general, you multiply the total amount by the percent to get the decrease in the amount. You can then subtract that decrease from the original amount to get the new result. Another way of finding the resulting amount is to subtract the percent decrease from 100 percent and multiply this difference times the original amount.
To determine the net result or amount after applying a percent decrease, you use one of the following methods (either one works):
Total amount X Percent decrease = decrease in amount Total amount – decrease in amount = net result 100 percent – percent decrease = decreased percent Total amount X Decreased percent = net result
The Problem: A local store is going out of business and has advertised that all items are 60 percent off the original price. You buy a toaster oven that’s currently marked $49.95. What will you pay for the toaster oven after the store applies the discount?
Using the first method, earlier, you first multiply $49.95 X 0.60 = $29.97. This is the amount of the decrease. Next, subtract $49.95 – $29.97 = $19.98. Wow! Such a deal!
Now, using the second method, you subtract 100 percent – 60 percent = 40 percent. The item will cost 40 percent of the original cost. Taking $49.95 X 0.40 = $19.98. You get the same answer, of course.
You’re pretty happy with your purchase of the toaster oven until, the next day, you see that the same store is offering to reduce the Previously reduced price By another 20 percent. Does that mean that the new reduction is 80 percent, or is the reduction 20 percent of the previous 60 percent? Is there even a difference? Oh, yes, there is.
The Problem: What is the difference between an 80 percent decrease and a 60 percent decrease followed by a 20 percent decrease? For the sake of comparison, use an item that has an original cost of $100.
To see if there is a difference at all, do the computations two different ways: Find the price after an 80 percent decrease. Then go back to the original price and figure a 60 percent decrease followed by a 20 percent decrease.
Determining the cost of the item if there’s an 80 percent decrease in the price, you subtract 100 percent – 80 percent, giving you a final cost of 20 percent of $100. Multiplying 100 x 0.20 = $20. That’s the cost of the item with a straight 80 percent decrease.
Now, find the cost when there’s first a 60 percent decrease in cost followed by a 20 percent decrease in that result — 100 percent – 60 percent = 40 percent. Multiplying 100 x 0.40 = $40. A 20 percent decrease becomes 100 percent – 20 percent = 80 percent. And, multiplying, $40 x 0.80 = $32. That’s quite a bit different from the $20 when figured the other way.
Not that I’m beating up on this poor store that’s going out of business, but here’s another scenario to consider when dealing with percent decreases. Consider a less-than-scrupulous manager who advertises that prices are going to be decreased by 60 percent off the original amount. What you don’t know is that he changes all the prices the night before the big sale so that the 60 percent decreases result in the cost of the items all being the same as they were the previous day. How does he do the necessary math?
The Problem: A store advertises a 60 percent decrease in cost on all items. What does the price tag have to read so that an item costing $100 originally will still cost $100 after the 60 percent decrease?
Go back to the same process of subtracting the 60 percent from 100 percent to get a net cost of 40 percent. Then divide instead of multiplying to get the required price. Dividing $100 ■ 0.40 = $250. The price needed on the item is $250. When you apply a 60 percent decrease to $250, you get back to the original price of $100. Somehow, I think that the shoppers just may notice the price hike.
Matching socks
You have 40 socks in a drawer in your bedroom. are blue. How many socks do you have to take The power is out, though, and you can’t see what out of the drawer to be absolutely sure that you color the socks are. You know that 35 percent of have a matching pair of socks? You don’t care the socks are black and 65 percent of the socks which color, as long as they match.
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Making the discount count
You’re going to take advantage of the end-of-year discounts being offered at the local hardware store. You get a 25 percent discount on all items, but you have to figure in the 8 percent sales tax.
The Problem: You have purchases totaling $75.45 and get to the checkout counter. Your coupon says 25 percent off, but there’s an 8 percent sales tax, so the clerk tells you that she’ll just subtract the 8 percent from the 25 percent, leaving a 17 percent discount, which means that you pay 83 percent of the total before all this magic arithmetic. Is this right?
You sense a shell game going on. The numbers are flashing through your head like those visions of sugar plums. Take a deep breath, and do the computations yourself. First, find the reduced cost, and then add the sales tax. The discount is 25 percent off, so you’ll only be paying 75 percent of the original price. (Refer to "Decreasing by Percents," earlier in this chapter, for more on figuring the discounted price.) Computing 75 percent of $75.45, you get 0.75 X $75.45 = $56.5875 or $56.59. Now add on the 8 percent tax by multiplying $56.59 times 108 percent. (This is the same as finding 8 percent and adding that amount on to the price. See "Determining an increase with percents," later in this chapter, for more information on this process.) The computation is 56.59 X 1.08 = $61.12. The tax was 8 percent of $56.59 or $4.53.
How does this compare to the clerk’s suggestion of just giving you a 17 percent discount? Computing 83 percent of $75.45, you multiply 0.83 X $75.45 = $62.6235. You would pay $62.62, which is $1.50 more than you should.
Determining an increase with percents
A percent increase may involve your goal in productivity or the amount of rainfall one summer or the amount that a price is increased due to sales tax. In general, to find the new amount after a percent increase, you either determine the increase in the amount that results from the percent increase and add it to the original, or you add the percent increase to 100 percent and multiply by the new percentage.
To determine the net result or amount after applying a percent increase, you use one of the following methods:
Total amount X Percent increase = increase in amount Total amount + increase in amount = net result 100 percent + percent increase = increased percent Total amount X Increased percent = net result
The Problem: A sheep shearer figures that he can improve upon his average per day shearing of 100 sheep. He’s set a goal of increasing the number of sheep by 15 percent. How many sheep per day will he have to shear to make that goal?
Using the first method, he first determines that 15 percent of 100 is 0.15 X 100 = 15. Add 15 to the usual 100 sheep, and he sees that he needs to shear 115 sheep per day.
Using the second method, if the shearer adds 15 percent + 100 percent, he gets 115 percent. Multiply 100 X 1.15, and he gets 115 sheep.
Percentages of more than 100 percent have decimal equivalents that are greater than 1. Be careful when moving the decimal point. For instance, 250 percent = 2.5, 800 percent = 8 and 1,000 percent = 10.
Problems involving sales tax are pretty much just percent increase problems. They get even more interesting when you figure in both a percent decrease because of a sale price and also sales tax.
The Problem: You purchase new shoes that were advertised as being 25 percent off the original price. You look at the sales receipt and don’t agree with the total price. You suspect that the sales tax was computed on the original price of the shoes, not the sale price. The shoes were originally $120, and the sales tax is 8>4 percent. The amount you’re being asked to pay is $97.43. Is this the correct amount?
^ylAJV First, figure out the new price due to the decrease. Then figure the increase in ‘ |SK\ price due to the sales tax. For the decrease in price, multiply the original cost H> \f ) times 75 percent. If you need help in determining where the 75 percent came ^ttmjw From, go back to "Decreasing By Percents," earlier in this chapter. After you
Get the new, lower cost of the shoes, multiply that amount by 108.25 percent.
This percentage is the result of applying a percent increase and adding the
Sales tax percentage to 100 percent.
Doubling up on amoebas
A jar contains seven amoebas. These particular amoebas multiply so fast (split into two) that they double in volume every minute. If it takes
40 minutes for the amoebas to fill the jar (for it to be 100 percent full), how long did it take to fill half the jar (50 percent full)?
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The new price of the shoes is $120 X 0.75 = $90. Now, to figure the total cost with tax, take $90 X 1.0825 = 97.425. The amount on the bill is correct, when you round up to the higher penny. No error was made here. If the tax had been figured on the original amount, the total would have been $99.90. I get this amount by figuring the tax on $120 to be $120 X 0.0825 = $9.90 and adding that tax to the reduced price of the shoes. It doesn’t hurt to check — errors easily can be made when figuring discounted prices and taxes.
Tipping the Waitress without Tipping Your Hand
Did you know that the word Tip Is an acronym for To Insure Promptness? The word Tip May not have started out to stand for those words, but it does seem to fit the situation. Waiters and waitresses are at your mercy when it comes to tipping them properly, so you want to be able to compute their payment with a minimum of hassle and struggle — and do it accurately. Sometimes the restaurant makes it easy for you and adds on a 15 percent tip. Also, if you charge the meal, you’re given a nice slip of paper to do your addition on — if you know how much you want to tip. A bit of a problem arises when you use a discount coupon. All these things have to be taken into consideration — and I walk you through them all in this section.

Figuring the tip on your bill
Even when the service is questionable, most people leave a tip. The amount can serve to indicate to the waiter just what you think of the service. The most commonly used tip percentages are 10 percent, 15 percent, and 20 percent. The first and last are fairly easy to compute. But 15 percent, is a bit of a challenge.
Adding on 10 percent or 20 percent
To figure a tip of 10 percent — which means that you aren’t particularly impressed with the waiter’s work — all you need do is take the total and move the decimal point one place.
Multiplying by 10 percent is accomplished by moving the decimal point one place to the left. If you multiply 48 x 10 percent this is 48 x 0.10 = 4.80 or 4.8.
Figuring the tip in your head
When you’re in a situation where you’re going to give only a 10 percent tip (and I hope that the situation doesn’t come up very often), you can do a quick, neat calculation in your head to find the total amount — the tip plus the charge for the meal. This little method works because it’s a quick computation used to multiply any number by 11.
The method you use is to picture a 0 in front of the meal cost and add the adjacent digits together. If any of the pairs add up to more than
9, you’ll have to carry 1 to the next sum. For example, if your bill is $45.32, then picture in your mind 045.32. Start on the right end and add 2 + 3 = 5, 3 + 5 = 8, 5 + 4 = 9 and 4 + 0 = 4. Write these sums in reverse order, with the decimal point in the correct place, and you get 49.85 — the sum of the meal plus a 10 percent tip. This calculation is going to be off by a penny if the last digit in the meal cost is bigger than 5 — because of the rounding. But, for a quick computation, it works pretty well.
The Problem: How much tip do you leave if you’re going to pay 10 percent on a bill of $18.80? And what is the total payment after adding the tip?
Move the decimal in $18.80 one place to the left to get $1.88. Add $18.80 + $1.88 to get a total of $20.68.
To figure a 20 percent tip, all you do is figure the 10 percent tip and double it. You can do this by either doubling the cost and then moving the decimal point, or you can figure the 10 percent amount by moving the decimal and then doubling that amount for the tip.
The Problem: How much tip will you give the hairdresser if the charge for a haircut and perm is $85 and you want to give a 20 percent tip?
JcVLAilf When you’re moving the decimal point in 85 one place to the left, you have to remember that there Is A decimal point in the number. The decimal point is always assumed to be at the far right of the number, if it isn’t showing. So one place to the left gives you 8.5 — which is $8.50. Double that to get $17. Add that tip to the bill for a total of $85 + $17 = $102.
Computing a 15 percent tip
Multiplying a number by 15 isn’t the hardest thing to do, but it isn’t quite as sweet as multiplying by 10 or 20. So it won’t come as a shock to find that figuring a 15 percent tip is a bit more involved than figuring a 10 percent or 20 percent tip.
The simplest approach is just to multiply the amount by 15 percent and be done with it.
VLAiV
The Problem: How much do you tip the waiter if the bill is $164, and you’re going to tip 15 percent? What is the total bill after the tip is added?
Multiply 164 X 15 percent = 164 X 0.15 = $24.60. So, adding $164 + $24.60, you get a total of $188.60 for the bill.
If you like to figure out the tip in her head, there’s a trick to computing the amount of the tip when you want to leave a tip of 15 percent. First figure the 10 percent tip by moving the decimal point one place to the left. Then take half of that tip and add it on to the whole tip. There’s your 15 percent tip — 10 percent and half of 10 percent.
The Problem: You’ve gone to lunch with nine of your friends and you’ve all agreed to just split the whole bill equally — ten ways — and leave a 15 percent tip. You’re in charge of collecting everyone’s money and then settling up at the cash register. The charge for the ten lunches is $188. How much will each person give you for her share?
You first have to figure the tip, add it on to the cost of the meal, and then divide the total by 10. You’re going to do this in your head — and hope you can do it correctly, or you’re going to get stuck with any shortfall. A 15 percent tip is 10 percent plus half of 10 percent, so it’s $18.80 + $9.40 = $28.20. Add the tip to the bill to get $188 + $28.20 = $216.20. To divide that by 10, you just have to move the decimal point again and get that everyone’s share is $21.62. Of course, what are the chances that everyone will have the correct change? Oh, sure.
Taking into account the discount
Everyone just loves those buy-one-get-one-free coupons or the percentage off the total discounts. These promotions get you in the door and are good for everyone involved. Sometimes you have to pay tax on the amount before the discount, and sometimes you pay tax on the lesser amount. It depends on what the product is — and if the merchant knows how to figure it correctly. That’s why you need to be aware of what’s going on so you can check the computations.

The Problem: Your favorite restaurant is offering a free second entree, as long as it costs less than the entree you’re paying for. This is how most of the buy-one-get-one-free promotions work. You’ll pay tax on the reduced price, but you need to tip the waitress based on the total cost before the discount. Your entree costs $19.95, and your friend’s entree costs $21.95. You’ve ordered beverages totaling $16 and shared an appetizer that costs $6.95. The tax (sales plus restaurant tax) comes to 10.5 percent, and you want to give a 20 percent tip. You and your friend will split the bill. How much will each of you pay?
You’re going to pay tax on all the items except the less-pricey meal, so add up all the items — food and beverages — except the $19.95 and compute the tax on that. You’re going to figure the tip on all the items except the tax, so you’ll need a different sum to do that computation. Last, you’ll add up the cost of the items you’re paying for, the tax, and the tip. Divide that total by 2, and you’ll have the amount that each of you owes.
First, computing the tax, add $21.95 + $16 + $6.95 = $44.90. The tax on that is $44.90 X 10.5 percent = 44.90 X 0.105 = $4.7145, making the tax $4.71.
Next, to compute the tip, add up the cost of all the items, $19.95 + $21.95 + $16 + $6.95 = $64.85. Multiplying $64.85 X 20 percent = 64.85 X 0.20 = $12.97.
Your total cost is the cost of the meals plus the tax plus the tip. Add $44.90 + $4.71 + $12.97 = $62.58. Divide that by 2, and each of you owes $31.29. Now, I know that in practice, most people would round the tip up to $13, but that doesn’t really change the amount by much.
KlSS: Keeping It Simple, Silly — with Simple Interest
Simple interest Is the interest computed when Compounding Doesn’t occur. The interest in a savings account Compounds, Because, if you don’t withdraw any of the money you’ve invested, your interest earns interest. With compound interest, the amount of interest earned is added to the account total, and then the new interest is figured on the new total. Simple interest is computed only on the beginning amount.
The formula for Simple interest Is I= prt, Where IIs the amount of interest earned, P Is the principal or amount of money involved, R Is the interest rate (a percent changed to a decimal for the computation), and T Is the amount of time involved — usually a number of years.

Determining how much interest you’ve earned
Problems involving Interest Are two types: interest earned, and interest you have to pay. The interest earned is the more-fun type. You get to add money to your savings account without even working at it.

The Problem: How much simple interest is earned on $10,000 if this money is deposited for 6 years in an account that earns 4>4 percent interest?
Using the formula for simple interest and replacing the letters with their corresponding values, you get I= $10,000 X 0.0425 X 6 = 2,550. You earn $2,550 in interest, so now the total in the account is $12,550.
The Problem: How much simple interest is earned on $10,000 if you have it in one account for 6K years at 4 percent interest and then move that money and the interest it’s earned to another account earning 6 percent interest for another 332 years?
To determine the total amount of interest earned, you have to add the two different interest amounts. One amount comes from the interest earned at 4 percent, and the other amount comes from the interest earned at 6 percent.
Apply the simple-interest formula on $10,000 at 4 percent for 634 years with I= 10,000 X 0.04 X 6.5 = $2,600.
Next, apply the simple-interest formula on $12,600 at 6 percent for 334 years. Why is the amount of money different? Because the total amount from the first 634 years, the principal plus interest, is all deposited in the new account. You get I= $12,600 X 0.06 X 3.5 = $2,646.
Now, to answer the question, "How much interest is earned?", you add the two interest values together — $2,600 + $2,646 = $5,246.
Figuring out how much you need to invest
The simple-interest formula, I= prt, Gives you the amount that your money has earned for you over a particular period of time. You have a set number of dollars and invest it for a chosen number of years. Of course, the longer you invest the money, the more interest you’ll earn. Another situation is that you may have a target amount of interest or a target total of money, and you need to know how much to invest right now to reach that target. For example, you may want to set up an account (usually called an Endowment) Where only the interest is spent each year while the amount in the account stays the same and is never withdrawn. Or you may want to have a particular total amount of money to buy a boat in ten years and need to make a deposit today so that the money in the account will grow to that target amount over the years.
Spending only the interest
A benefactor wants to donate money to a local charity but doesn’t want the charity to spend it. How does this help the charity? The arrangement is that the charity gets to spend only the interest, every year, and the amount in the account stays there to earn interest the next year and the next and so on.
Inheriting a fortune
A sheikh had two sons who were equally likely the first to the city, because each wanted his
To inherit his position and his fortune. He camel to be the slower one. They happened
Devised a contest to see which son would be his upon a wise man and asked him for advice on
Successor. In this contest, the two sons would how they could finish this contest. The wise
Race their camels to a distant city. The winner man gave it his best. After hearing what he had
Would be the one whose camel was Slower! To say, the two sons jumped on the camels and
The sons started the race and wandered about aimlessly for several days; neither wanted to be
Raced as fast as they could to the city. What did the wise man tell them?
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The Problem: How much money must be invested in an account that earns 5 percent simple interest per year if the interest must come out to be $6,000?
.VLAiV Use the simple-interest formula, I = Prt, Replacing the letters with the corresponding values in the problem. You’ll be solving for P, The principal. The equation becomes: $6,000 = P x 0.05 x 1 or $6,000 = 0.05p. Divide each side of the equation by 0.05 to get P = $120,000. The person donating the money needs to put $120,000 in the account for there to be $6,000 of spendable money (or interest) each year.
Aiming at a future purchase
You have your eye on a new powerboat — one that seats up to 12 people and moves fast enough to pull a skier. You figure that the boat you want will cost close to $100,000 when you’re ready to buy it. You’re going to put away a lump sum of money today, and let it grow in value for ten years — at which time you’ll take all the money and the interest to buy the boat.
The Problem: How much money do you have to deposit in an account earning 8 percent simple interest if you want to have a total of $100,000 in principal and interest in ten years?
^jVLAiV If you use the simple-interest formula, I = prt, you’ll get varying amounts of interest, depending on what the principal is. What you want is for the principal, P, And the interest, Prt, To have a total of $100,000. Your equation would look like: P + Prt = $100,000. You can solve for the principal by dividing the $100,000 by 1 + Rt. Just add 1 to the product of the rate times the time and divide that into the $100,000. You’ll be dividing $100,000 by 1 + (0.08 x 10), which is: $100,000 1.8 = $55,555.56. Actually, the money grows even faster if you use compound interest, but this is pretty impressive just as it is here.
Working out the payments
Many people use credit cards to pay for large purchases, but some stores still offer convenient short-term payment plans on their merchandise. If you want to buy an item, the store figures out the total cost of the item plus the interest over a period of time, divides the total into equal payments, and then lets you purchase the item paying back the same amount for a certain number of months or years.
The Problem: You want to buy an all-leather sectional sofa that costs $3,500. The store will let you pay for it over the next 36 months at 9 percent interest. How much will your monthly payment be if you’re going to be paying for the sofa and three years’ simple interest in equal monthly payments?
Figure out the interest on $3,500 at 9 percent for 3 years. Add the interest to the cost of the sofa and divide the total by 36. The interest is $3,500 X 0.09 X 3 = $945. Add $3,500 + $945 to get $4,445. Divide $4,445 ■ 36 = $123.47222. . . . You can’t just lop off the remainder. The 0.00222 . . . represents a remainder of 8 cents in the division, so you’ll pay $123.48 for 8 months and $123.47 for the other 28 months.
How do you get the 8<t remainder? You multiply $123.47 by 36, and you get $4444.92, which is 8<t short of the total. Long division gives you a remainder of 8<t. Calculators give you the decimal. And, if you’re so lucky, you have a graphing calculator that changes decimals into fractions automatically.
Chapter 7
In This Chapter
^ Using proportions to figure fair shares ^ Working with proportions effectively ^ Weighing all the choices
R

Roportions are nothing more than two ratios or fractions set equal to one another. Proportions have several very handy properties that make working with them much easier to manage. In this chapter, you see how to set up the proportions correctly and how to solve the problems you’ve created with the proportions. You see how to apply the properties of proportions to make the solutions easier.
Working with the Math of Proportions
A proportion is an equation involving two fractions. The ratio of the numerators and denominators of the fractions must be equal. A proportion is a statement saying that two fractions are equivalent or equal in value. The fractions can be reduced in the normal way — the way you’ve see since third grade — and they can be reduced in some rather unique ways, too. You use this prop-^ABEft Erty and several others to solve for unknown parts of a proportion.
Given the proportion % = c, the following also are true:
A x d = B X C The cross products are equal.
% = D The reciprocals are equal.
You can reduce (eliminate common divisors):
% = e # g, % = e——, % = 7T You can reduce the fractions
B e #g b e # g b g
C
E X F e XG> EX F e#G’ F g
Vertically.
You can reduce the fractions horizontally.
%
Solving proportions by multiplying or flipping
Equations involving proportions are solved using the properties of proportions. When you cross-multiply, you get rid of the fraction format, which gives you an equation that is usually simpler to deal with. Also, if you flip the proportion, you make the problem more to your liking — easier to solve.
Cross-multiplying in a proportion
You can solve for the value of X In the proportion <2x +i3 = 5 by cross-multi -
1 • 1 • 1 C TI c t – iri. X — 3 7 J
Plying and getting rid of the fractional format.
2x + 3 = 5 X — 3 7 (2x + 3) X 7 = (X — 3) X 5
14x + 21 = 5x — 15
9x =—36 X=—4
Flipping your lid over a proportion
Even though cross-multiplying is a great tool to use when solving proportions, you can often take an easier route: Flip the fractions (set the reciprocals equal to one another) and then multiply each side by the same number to solve the equation. For example, in the following equation, I flip the proportion and then just have to multiply each side by the number under the X, Reduce, and get the answer.
70 35
— = —
X 21 X 21
= ——
70 35 X=21 X 2 =42
Going every which way with reducing
Reducing fractions in proportions is a blast! You can reduce across the tops, across the bottoms, up and down on the left, or up and down on the right. You just can’t reduce going diagonally — the crisscross motion is for multiplying only. By reducing the proportion, first, before cross-multiplying, you get to work with smaller numbers. That’s a good idea not only for the ease of the problem, but also because it helps prevent errors.
Reducing vertically
Proportions are created from fractions, and the traditional way of reducing fractions is to find a number that divides the numerator and denominator evenly and divide each part of the fraction by that number. Reducing fractions is sometimes referred to as Cancelling. In the following proportion, the numbers in the fraction on the left are each divisible by 5. After reducing the fraction on the left, you can cross-multiply and solve the equation for X.
25 X
— = —
40 16 25 X
- = -
5X
— =
8 16 5 X 16 = 8x
80 = 8X
10 = X

You could also have reduced horizontally in this equation, because 8 and 16 are both divisible by 8. The next section shows you how that works.
Reducing horizontally
The property of proportions that allows you to cross-multiply and have a true statement is the same property that allows for reducing across the proportion horizontally. Both of these clever tools are due to the commutative property of multiplication — the fact that reversing the order of the numbers in a multiplication property doesn’t change the answer. It’s just neat that this property comes in so handy when working with proportions. In the following proportion, the two numerators are each divisible by 11. Then you see that the two numbers in the right fraction are divisible by 7. Reduce that fraction so that, when you cross-multiply, you don’t have to multiply by 63.
X 63 27
— zz -
X 63
71
63
2X9 XX1
18 X
Dividing $17
Sneaky Pete thought that he could pull one over on his buddies and keep all the money that was paid to all of them for taking care of the neighbor’s dog. The neighbor gave Pete $17 and told him to divvy it up according to how much time each friend spent taking care of the dog. Pete took the money and posed this problem: "In my hand, I have $17. Tom can have one-ninth of the money, because he worked one-ninth of the
Time. Tim can have one-third of the money, because he worked one-third of the time. And Ted can have one-half of the money, because he worked half of the time. You aren’t allowed to tear any of the dollar bills, and there’s no change. If you can’t figure out how to divide up the $17, then I’ll just keep it all." Pete’s buddies were just too smart for him and divided up the money equitably. How did they do it?
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Dividing Things Up Equitably
Everyone should play fair. Whether it’s in basketball or a card game, you expect to be treated fairly. The same can be said for sharing candy or money or time. If everyone gets an equal share (all the same amount), then the computation is easy — you just divide by the number of people who are involved. It gets a little more complicated when the shares are to be unequally divided, like someone getting twice as much as another because she did twice the work.
Splitting things between two people unevenly
If two children are to share equally in an inheritance, you just divide the total amount by two. It gets a bit stickier when one person gets more than the other. It strains the family relationship %nd The mathematics.
The Problem: Henry and Hilda are to share their father’s $2.5 million estate. Their father said that Henry is to get 65 percent of the estate and Hilda is to get the other 35 percent. How much does Henry get?
Using proportions, think of Henry’s 65 percent as being 65 out of 100, where 100 is the total amount. Write the proportion with X Being the unknown
Amount out of $2.5 million. The proportion is = 2 500 000. The proportion says that 65 out of 100 is equal to some unknown value out of 2,500,000 (2.5 million).
When writing proportions, put Related Amounts either horizontally or vertically from one another. Units that are alike should be either across from one another or above and below one another.
In the problem, the units that are Related Are the $2,500,000, which is All Of the inheritance and the 100 which represents 100 percent or All. The X And the 65 each represent a Part Of the whole thing. To solve the proportion and determine Henry’s share, first reduce across the bottom of the proportion, and then cross-multiply and solve for X.
65 X
-= -——-
100 2.500:000 ,
1* —25000
65 X
1 — -
25,000 65 X 25,000 = X x 1 1,625,000 = X
Henry gets $1,625,000 of the $2.5 million in their father’s estate.
Figuring each person’s share
Another possible scenario when dividing things up is that three or more people are involved, and they each get a different share or fraction of the total amount. A situation like this occurs when people do different amounts of the total work or when they are different ages or different weights or whatever makes them different from one another.
The proportion or proportions used to solve problems where three or more people get differing shares all have a common theme. You’re always concerned with the total amount — and all the parts must add up to the total amount.
If a pie is to be divided among four people, and the shares are one-twelfth, one-sixth, one-fourth, and one-half of the pie, you have to be sure that these fractions all add up to 1 — which is the whole pie.
1 + J_ + J_ + J_ = J_ + _2_ + _3_ + _6_ 12 6 4 2 12 12 12 12
= 1+2 + 3 + 6 = 12 = 1 12 12
Consider a situation where contestants share in the total prize depending on the number of points that they’ve scored.
The Problem: In a fishing tournament, a sports-equipment company has offered a prize of $100,000 to be divided among the top five winners in proportion to their scores. The points earned by the top five winners are: 60, 40, 30, 20, and 10. How much money does each get?
Determine the total number of points, and then figure out the proportion or part of the prize that each gets. The total number of points is: 60 + 40 + 30 + 20 + 10 = 160 points.
Writing a proportion with the 60 points for the top scorer, = Tjtt^aa . N i ti i ^ it – i iii 160 100,000
Reduce across the bottom, cross-multiply and solve for X.
60 X
160 100000 ,

\s -— 625
60 X625 = 1 XX 37,500 = X
The top scorer gets $37,500. The rest of the winners get amounts as shown here:
The second-place winner gets $25,000, determined from
40 = X 40 = X 4x = 100 000 160 100,000 160 100,000 ’ ‘
The third-place winner gets $18,750, from
30 = X 30 = X = 300 000
160 100,000 160 100,000 ’ ‘
16
The fourth-place w1inner gets $12,500, using
20 = X 20 = X 8x = 100 000 160 100,000 160 100,000 ’ ‘
‘ 8
The fifth-place winner gets $6,250, which you determine by just subtracting all the other prizes from $100,000 and seeing what’s left.
Comparing the proportions for differing amounts of money
Some very interesting problems occur when an estate is divided up between all the heirs. And then there’s the executor’s share to be considered in the mix. Proportions are very handy when you’re determining who gets how much money.
Diophantus’s life
Diophantus was a Greek mathematician who had an impact on algebra back in the day. A puzzle that’s attributed to his life goes: "Diophantus’s boyhood lasted for one-sixth of his life. He married after one-seventh more. His
Beard grew after one-twelfth more of his life, and his son was born 5 years later. The son lived to half his father’s age, and Diophantus died four years after the son. How long did Diophantus live?
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The Problem: A woman’s estate totals $5 million. She leaves KO of her estate to her butler, %> to her chauffer, XO to her daughter, !4 To her son, and KO is to be put in a trust to take care of her dog, Puddles. But all these bequests aren’t to be made until After The executor of her will gets 10 percent of the original amount. The others get their share of the net amount. How much does each person get?
■AVLAW You first deduct the amount that the executor gets. Because she gets 10 percent of the estate, you multiply $5,000,000 X 0.10 = $500,000. Subtract that from the estate, and it leaves $4,500,000 to be divided among the others. The other shares are determined using proportions, letting the fractional share be one side of the proportion and an X Divided by $4,500,000 be the other side of the proportion. The value of X In each case is that person’s share.
Butler: -XR = . ,Ax AAA, – Xr = . „ fL^-, X = 225,000
20 4,500,000 20 4,500:000
‘ 1 ^jj-—-’ 225,000
Chauffer: -k^ = . ,Ax AAA, – Mr = , rr, A -, X = 3 • 225,000 = 675,000
20 4,500,000 20 4J500000
1 225, 000
Daughter: Mr = A r^ nnn, "^r ^ ^-, X = 450,000
0 10 4,500,000 10 4.51M0000
Son: = 4 500 000, "4~ = 4 5OO-00t)-, X = 1,125,000
‘ ’ * 1 ^.JJ-1*"7, 1,125,000
Puddles: – J77 = Tft^h, -Mr = . rAA -, X = 9 $ 225,000 = 2,025,000
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20 4,500,000 – 20 4,500000
12

Comparing Apples and Oranges
One of the nicest things about proportions is that they can be used to solve problems involving items that don’t seem to have anything in common except for their ratios to one another. For instance, if you’re told that three apples can be traded for four oranges, then you can figure out how many apples you can get for 28 oranges by using a proportion.
The Problem: How many apples can you trade for your 28 oranges if the current trading rate is that three apples are worth four oranges?
Write the apples over the oranges in one fraction, and then put the 28 oranges on the bottom of the other fraction — opposite the oranges in the first fraction. Let the unknown number of apples be represented by X.
3 apples _ X Apples 3 _ X
4 oranges 28 oranges 4 28
This proportion is solved by reducing across the bottom and cross-multiplying.

3 = X 21 = X 41 287′
It takes 21 apples to get 28 oranges. As long as you have apples across from apples and oranges across from oranges, the proportion will work. Another format for this is to have apples over apples and oranges over oranges, with the equivalence of 3 and 4 across from one another. (Refer to "Working with the Math of Proportions," earlier in this chapter, if you need help with the manipulations of proportions.)
Determining the amounts in recipes
If you’re a cook — or even if you don’t have much interest in the culinary arts — you’re apt to come across a situation where you need to double a recipe for a bigger crowd, halve a recipe that makes too much food, or compute some such multiple or part of a recipe. Even if the recipe is for cement, getting the amounts correct is important. Otherwise, the chili will taste too salty or the cake won’t rise or the cement will never harden. Proportions are a huge help with these recipe challenges.
The Problem: Your favorite chili recipe calls for 2 pounds of hamburger and 3 onions. You’re going to make enough chili for your whole fraternity and plan to use 28 pounds of hamburger. How many onions do you need?
^VLA* Set up a proportion with pounds of hamburger divided by number of onions in one fraction and pounds of hamburger divided by onions in the other fraction. Place an X For the unknown number of onions. Be sure to put the 28 pounds of hamburger in the same fraction as the X Number of onions.
2 pounds _ 28 pounds 2 _28
3 onions X Onions ‘ 3 X
Solving this proportion is easier if you reduce across the top by dividing each numerator by 2. Then flip the proportion before cross-multiplying. (Refer to "Working with the Math of Proportions," earlier in this chapter, if you need a refresher on these techniques.)
H = .2814 3 = X_ = =
3 X 1 14
You will need 42 onions. I feel a crying session coming on.
I inherited my grandmother’s recipe box, and there are some wonderful, old recipes in it. One of my favorites (just because I’m trying to imagine my very proper grandmother, Marion Jones Roby Ingersoll, making this much food) is for sausage. The recipe calls for 100 pounds of pork, >4 pound of sage, >4 pound of pepper, 2 pounds of salt, 1 tablespoon of mustard and a "little" summer savory. The recipe says to put the stuff in a crock by layers and "weigh it down."
The Problem: I want to try my grandmother’s recipe, but not in that huge quantity. If I start with 5 pounds of pork instead of 100, how much sage and mustard will I need?

^VLA* Even though the quantities are in pounds and tablespoons, I can still use proportions to solve for the amounts needed. Write a proportion with the original number of pounds of pork and sage in the numerator and denominator of the first fraction, and then write the 5 pounds of pork in the other fraction across from the 100 pounds. Solve for the reduced pounds of sage. Do the same thing with the original measures to solve for the amount of mustard.
100 pounds = 5 pounds 100 = 5 20 = 1 = 1 1 pound p ^
When I divided each side by 20, I got >8o pound of sage. It makes more sense to change the fraction of a pound to ounces. If there are 16 ounces in a pound, then multiply 16 X >8o pound to get >5 ounce. And now, for the mustard, write a new proportion and solve it.
100 pounds = 1 T 100 = 1 100 = 1 = 1 4 pound 4
The measure of >4oo of a tablespoon is probably better known as a "dash."
Truck on a bridge
A large truck is crossing a bridge that measures of the truck and all its cargo. The truck makes it 1 mile in length. The bridge can only hold 14,000 halfway across the bridge and stops. A bird pounds, which happens to be the exact weight lands on the truck. Does the bridge collapse?
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Figuring out weighted averages
Weighted averages are used to give more importance or emphasis to one thing than another. A prime example of weighted averages is when they’re used to determine your grade in a college course. The weighting can go something like this: Tests count three times as much as papers, the final exam counts twice as much as a test, and attendance counts one-fourth as much as a paper. In general, to find a weighted average, you set up a proportion and multiply the weights times their respective amounts. Look at these next two problems, and you’ll see what I mean.
The Problem: An Astrodollars Coffee shop sells several different types of whole coffee beans. Last Monday they sold 100 pounds of Honduran coffee beans, 70 pounds of Guatemalan coffee beans, 40 pounds of Nicaraguan coffee beans, and 40 pounds of Chilean coffee beans. The Honduran beans cost $8 per pound, the Guatemalan beans cost $9 per pound, the Nicaraguan beans cost $10.50 per pound, and the Chilean beans cost $13.50 per pound. What was the average cost per pound of the coffee beans sold on Monday?
£?LAiV Set up a proportion where each poundage multiplies its respective price. Put ‘ <ES\ the sum of all the products you get in the numerator of a fraction, and divide Hp \f ) by the total number of pounds. That’s one side of the proportion. Set that
Fraction equal to another fraction with a 1 in the denominator (opposite the
Total number of pounds) and an X In the numerator.
(100 X
+ (70 X 9.00) + (40 X 10.50) + (40 X 13.50) = X 100 pounds + 70 pounds + 40 pounds + 40 pounds 1 pound
Now simplify the proportion by doing the multiplications and additions; then reduce the fraction on the left. Cross-multiply and solve for X, And you get $9.56 for the average cost of the coffee beans sold on Monday.
800 +630 +420 +540
100 +70 +40 +40 1 pound
2,390 X
=
250 1
X
2
2,390 250 1
25
239 = 25X 239 X = "25"
9.56
A college grade point average (GPA) is usually a weighted average. Different courses are a different number of hours or quarters or other units, which serve to determine their relative worth. Consider a student who attends a college that measures courses in semester hours and uses grades of A, B, C, D, and F. The corresponding point values for the grades are: 4, 3, 2, 1, and 0 points.
The Problem: Nick took five courses last semester and needs to determine his GPA. He got an A in his 4-semester-hour calculus course, an A in his 1-semester-hour computer course, a B in his 5-semester-hour biology course, a C in his 3-semester-hour English course, and a B in his 3-semester-hour Spanish course. What is his GPA for the semester?
Multiply each number of semester hours by its worth in terms of points for that grade, and add up all the points; divide by the total number of semester hours. Set that fraction equal to X Divided by 1.
(A x 4) + (A x 1) + (B x 5) + (C x 3) + (B x 3) = X 4 + 1 + 5 + 3 + 3 1 (4 X 4) + (4 X 1) + (3 X 5) + (2 X 3) + (3 X 3) = X 16 1 16 +4 +15 +6 +9 X
-t~7*- = ~t~
16 1
50 X

— = ——
16 1
25
50 X
16 1
25 8X
8
3.125 = X
X
After simplifying the proportion, reducing the fraction on the left, cross-multiplying, and solving for X, You get that Nick’s GPA is 3.125.
Computing Medicinal Doses Using Proportions
Modern medicine offers many wonderful options to reduce pain and suffering. The amount of the medication for a particular person has to be correct, though. The amount of medicine prescribed may depend on a person’s weight, age, or current health status — or, often, a mixture of all these things. Proportions are used to determine dosages of many medications and the number of tablets needed per dose.
Figuring the tablets for doses
A Scored tablet Is a medicine tablet that is designed so that it can be broken into halves or quarters, making it possible to administer a dose that is less than the amount in the tablet. For purposes of these problems, assume that the tablets are scored into quarters. (You can break the tablet into four equal pieces.)
The Problem: A doctor prescribes 0.375 mg of Digoxin, and the scored tablets that are available contain 0.25 mg each. How many tablets should be administered?
Set up a proportion with the amount per tablet in one fraction and the needed dosage and number of tablets in the other fraction.
0.25 mg 0.375 mg
—
- = -
1 tablet X Tablets
Reduce the fractions across the top by dividing each numerator by 0.125. Then solve for X.

0252 = 03753 1 = X
2X= 3
X= 1.5
The patient needs 1>2 tablets.
The Problem: A patient is to take 0.5 g of Ampicillin, and the capsules available are 250 mg. How many capsules are needed?

From the metric system: 1 gram = 1,000 milligrams or 1 g = 1,000 mg.
Change the 0.5 grams to milligrams using a proportion with 1 gram over 1,000 milligrams in one fraction and 0.5 grams over X Milligrams in the other fraction. Solve for X.
1g 0.5 g
- _ -
1,000 mg X Mg
1 _ 0.5 1,000 X
X _ 0.5 X 1000 _ 500
So 0.5 g = 500 mg. If the patient is to take 500 mg of Ampicillin, and the capsules are 250 mg, then the patient will need two capsules.
Making the Weight count
A person’s weight can affect the dosage of the medication they’re given. You use a proportion to determine the amount of medication based on that weight.
The Problem: Robert has been taking 80 mg of a medication every day. When it was prescribed, he weighed 170 pounds. He’s lost 30 pounds, so how much should the new dosage be?
Create a proportion with 80 mg and 170 pounds in one fraction and 140 pounds in the other fraction. The 140 is found by subtracting 170 – 30 to determine Robert’s new weight. Be sure to put the 140 pounds opposite the 170 pounds in the proportion.
80 mg X Mg
- zz -
170 lb 140 lb
Reduce either vertically in the left fraction or horizontally across the bottom. Then cross-multiply and solve for X.
You can "drop" the zeros in only one direction. Reducing by dividing by 10 in the proportion can be done either vertically or horizontally, but not both.
80 X
- zz -
170 140
80 X
=
170 140
S 17 * 14
1,120 17
The dosage is about 65.88 mg of the medication. The doctor will have to round up or down to find a suitable tablet to use.
Chapter 8