A percent tells what part of a whole you want, out of 100 (see Percent Means per 100). To find 30% of 80, think of 80 split into 100 equal tiny parts and take 30 of them.
Key idea
To find a percent of a number, find 10% (or 1%) of it first, then build up. Or multiply: $p\%$ of $n$ is $\frac{p}{100} \times n$.
Why it works
Taking 30 of the 100 tiny parts of 80 is $\frac{30}{100} \times 80$. Splitting into 10 equal parts first is a friendlier path to the same place: 10% of 80 is $80 \div 10 = 8$, and 30% is three of those tenths, $3 \times 8 = 24$. Both ways give 24, because $\frac{30}{100} = 3 \times \frac{1}{10}$.
Worked example: Build from 10%
Find 30% of 80.
- 10% of 80 is $80 \div 10 = 8$.
- 30% is 3 times as much: $3 \times 8 = 24$.
Answer: 24
Worked example: A tip
A family leaves a 15% tip on a \$40 meal. How much is the tip?
- 10% of \$40 is \$4.
- 5% is half of 10%: \$2.
- 15% = 10% + 5%: $\$4 + \$2 = \$6$.
Answer: \$6
Worked example: Use a fraction
What is 75% of 36?
- $75\% = \frac{75}{100} = \frac{3}{4}$.
- $\frac{1}{4}$ of 36 is 9, so $\frac{3}{4}$ of 36 is $3 \times 9 = 27$.
Answer: 27
Common mistake
Dividing by the percent: "30% of 80" is not $80 \div 30$. Quick check with size: 30% is less than half, so the answer must be less than half of 80, which is 40. And 100% of a number is the number itself, so 120% of 80 must be more than 80.