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Chapter 3 · Decimals and percents · page 17 of 68

Percent of a number

levelPre-algebra
standard6.RP.A.3c
practiceDecimals and percents

The word "of" means multiply. 20% of 80 is 0.20 times 80. The same multiplication, turned around, answers "what percent" and "percent of what".

How to use it

  1. Turn the percent into a decimal.
  2. Multiply it by the number.
  3. What percent: part divided by whole, times 100.
  4. Percent of what: part divided by the percent as a decimal.

Example

20% of 80=0.2×80=1620\% \text{ of } 80 = 0.2 \times 80 = 16
15 is what percent of 60:  1560=0.25=25%15 \text{ is what percent of } 60: \; \frac{15}{60} = 0.25 = 25\%
30% of what is 12:  120.3=4030\% \text{ of what is } 12: \; \frac{12}{0.3} = 40

Where it goes wrong

  • Multiplying by 20 instead of 0.20.
  • Dividing whole by part when asked what percent. The part goes on top.
  • Using 10% shortcuts carelessly: 10% of 80 is 8, so 5% is 4 and 15% is 12. Good when done on purpose, wrong when done by accident.

Related

Percent means per hundredPercent change and discountsProportions and cross multiplying
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