MathBench > Miscellaneous

Tricks with Division

Absolute amounts from percents

Every day the diet websites publish new recipes… Yesterday’s lowfat pound cake contained 21% butter. Maybe tomorrow I’ll find one that has only 18% butter. So how much butter would that be?

Well, first of all, you should be asking me, “18% of what?” If it’s a four-pound cake, then it will have a different amount of butter than a one-hundred-pound cake. Let’s stick with the “classic” 4 pound (64 oz) cake.

Next, think about what the word “of” means.

If you say, “five of something”, it means multiply the something by 5.

If you say, “one-half of something”, it means multiply the something by 1/2.

If you say, “18% of something”, it means multiply the something by 18%.

So as a general rule, replace the word “of” by the word “times”.

Here are two ways to do this calculation:

  1. Convert the percent back to a proportion, and remember that “of” means “times”, so 18% of 64 oz = 0.18 * 64 = 11.5 oz
  2. Convert the percent to a fraction, and remember that “of” means “times”, so 18% of 64 oz = (18 / 100) * 64 = 11.5 oz

In any case, you’ll come up with the same answer.

More quick practice

lower-fat-2How much of each ingredient is in my new-new-lowfat recipe for a 64 oz pound cake?

18% soy-oil butter

16% fake eggs

25% flour

41% sugar

Final note: the “of” trick works on proportions and fractions as well. So,

2/3 of 5/8 = 2/3 * 5/8

0.55 of 5/9 = 0.55 * 5/9