Math for Grownups (3 page)

Or, in mathematical terms,

37%
=
= 0.37

Converting percents to fractions or decimals can be really helpful—and really easy.

Convert 25% to a fraction:

Remember,
percent
means “out of 100,” so 25% is 25 out of 100, or
.

But now you need to simplify. What is the biggest number that will divide evenly into both 25 and 100? Divide the top and bottom numbers of the fraction by that number like this:

Thus 25% is the same thing as ¼. (But you probably already knew that.)

Convert 25% to a decimal:

When you convert percents to decimals, you’re actually dividing the percent by 100.

25%
=
25 / 100
=
0.25

But there’s a pattern here that makes things a lot easier. All you are doing is moving the decimal point two places to the left. (Check it with your calculator, if you’re suspicious.)

25%
→
0.25

There’s no decimal point in 25%, you say? Actually, there is. All numbers have decimal points; if you don’t see them, it’s just because they’ve been dropped, and you can put them in at the far right of the number. So the decimal point in 25% is on the right side of the 5, for 25.00. Move it two spaces to the left to produce 0.25.

In the late 1800s, C. W. Post began giving out tickets for 1¢ off the price of his new breakfast cereal, Grape Nuts. Who knew that this idea would translate into billions of dollars in savings?

A penny here and a penny there can add up, but using coupons takes some dedication and time. And then there’s the question on every shopper’s mind: Is it worth it? A few quick calculations can help you find out. Here’s an example:

While reading through the Sunday newspaper, Georgia clipped a pile of coupons for her next trip to the local Piggly Wiggly. She also took a few moments to categorize them, and this is what she has:

What’s the easiest way for her to figure out how much she’ll be saving?

Using an estimation skill will probably help. If she groups like numbers, she can find out her savings in the time it takes to clip a coupon.

1 • $1.50
=
$1.50

1 • $1.00
=
$1.00

4 • 75¢
=
$3.00

4 • 50¢
=
$2.00

5 • 25¢
=
$1.25

Georgia can now add all of the dollar values to get $8 and all of the change to get 75¢. Her total savings will be $8.75. And on double-coupon day, she’ll have, well, doubled her savings to $17.50!

So, is this really a big deal? A little more arithmetic can answer that question. If Georgia shops on double-coupon day each week and averages $8.75 in coupons a week (doubled to $17.50), she’ll save $910 each year.

But there are times when using a coupon isn’t worth it. That’s when you need to “comparison shop.”

Georgia has a coupon for 25¢ off the price of That’s Italian! Italian salad dressing in the 14-ounce bottle. But her grocery store is advertising a sale on Little Italy Italian dressing. Each 14-ounce bottle is 10% off the regular price. If That’s Italian! is $3.95 per bottle and Little Italy is $4.10 per bottle before the sale, which is the better deal: the coupon or the sale?

To find out, Georgia needs to find 10% of $4.10, and subtract that from $4.10 to find the sale price of the Little Italy dressing.

10% of $4.10 is 41¢

$4.10
-
$0.41
=
$3.69

But how much will Georgia save if she uses the coupon to buy That’s Italian! dressing?

$3.95
-
$0.25
=
$3.70

In fact, she’ll save an extra penny by skipping the coupon and buying the dressing that’s on sale.

What about on double-coupon day? In that case, it makes more sense to use the coupon:

25¢ • 2
=
50¢

$3.95
-
$0.50
=
$3.45

Now that’s a deal!

It’s not like food manufacturers are out to trick you, but they don’t necessarily make it easy to figure out the best deal.

Comparison shopping would be much easier if all of the cereal boxes were the same size. Instead, cereals come a variety of different weights—from 12 ounces to 24 ounces. And that doesn’t count the bulk packages!

To find the best deal, you’ll have to calculate the price per ounce.

(Before you go any further, read the last sentence again. Did a particular word jump out at you?
Per
means “each,” as you relearned a few pages ago. And because of that,
per
should also make you think of division. Tuck that idea away for this next example.)

Every day, Jerry enjoys a big bowl of cereal for breakfast, lunch, and dinner. But his new Great Dane is putting a serious cramp in his dining habits. Now Jerry spends his hard-earned cash on the dog, which means he has to cut down on his cereal budget.

How can Jerry still feed his cereal habit, while saving money? Comparison shopping, of course. If he knows which cereals are the best buy, he can keep himself
and
Rufus happy.

The corner supermarket has three brands of corn flakes on the shelf: Kentucky’s Best, ’Ears to Health! and Flakes O’ Corn. Each brand is a different size, and none of them have the same price:

If Jerry considers the price per ounce, he can figure out the best deal. To do that, he should divide each box’s price by the number of ounces the box contains. (The word
per
was Jerry’s big clue that division is involved.)

Let’s start with Kentucky’s Best, which costs $3.59 for 12 ounces. If we divide $3.59 / 12, we come up with $0.29916 (going on forever). Rounding up, we have a price of 30¢ per ounce. Here’s what happens when you do that for each cereal:

Even though ’Ears to Health! is in the smallest box, it offers the lowest price per ounce.

One of America’s most expensive habits is drinking bottled water. If the average price of bottled water is $1 per 20 ounces, and you drink about 64 ounces of water every day, you’re spending about $3 per day on water. That’s $1,095 per year.

Still, drinking tap water may not be an option. In that case, consider a water-filtering system. A pitcher filtering system has a price tag of about $20, and each filter costs about $10. If you need to change the filter after every 40 gallons of water, how much water will you drink before you need to spring for a new filter? There are 128 fluid ounces in a gallon, so multiply to find out: 40 • 128. That’s 5,120 ounces. And if you drink 64 ounces of water a day, you can divide to find out that this is 80 days’ worth of water (5,120 / 64 = 80).

So the filter costs 12.5¢ per day, or $45.63 per year. Add in the cost of the pitcher, and you’re shelling out a whopping $65.63 each year for filtering your own water.

This is $1,029.37 less than you’d spend on bottled water. (Filtering your own water is nicer to Mother Nature, too.)

You’ve heard this axiom before: Bigger is not always better. And that goes for buying in bulk, too.

Here’s an example:

At Miller’s Market, the 114-ounce bottle of catsup is $11.98. The 32-ounce bottle is $3.29. Which is the better deal?

Let’s calculate:

$11.98 / 114 ounces = 10.5¢ per ounce

$3.29 / 32 ounces = 10.3¢ per ounce

The larger container is actually more expensive than the smaller one.

Remember all those times that your math teacher hounded you about putting the units in your answers? Raise your hand if you got a test question wrong because you wrote “14” instead of “14
pounds
.”