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Computing Percents

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Finding a Percent of a Number

People deal with percents all the time in everyday life. When sales clerks
compute sales tax, they compute "7% of your total purchase." Similarly, when
diners are deciding what size tip to leave, they compute "15% of the cost of our
food." How do they perform these calculations?

Just as with fractions, the word "of" in a percent problem indicates
multiplication. The sales clerk multiplies 7% by the total purchase, and the
diner multiplies 15% by the cost of her food.

To find a percent of a number, first change the percent into a decimal or a
fraction. Next, multiply that decimal or fraction by the number.

*Example 1*: What is 15% of 220?

I. Using decimals.

15% = 0.15 and 0.15×220 = 33

II. Using fractions.

15% = 15/100 = 3/20 and 3/20×220/1 = 3/1×11/1 = 33

*Example 2*: What is 12.5% of 52?

I. Using decimals.

12.5% = 0.125 and 0.125×52 = 6.5

II. Using fractions.

12.5% = 0.125 = 125/1000 = 1/8 and 1/8×52/1 = 52/8 = 13/2 = 6 1/2