Percent of a number
Percent means out of 100, so every percent is a fraction with 100 on the bottom. To find a percent of a number, change the percent into a fraction or a decimal, and then multiply it by the whole amount.
A worked example
For example, find 25% of 80. 25% means 25 out of every 100, which is the same fraction as one quarter, and one quarter of 80 is 80 ÷ 4 = 20. Next, find 30% of 150 by writing 30% as the decimal 0.30 and multiplying: 0.30 × 150 = 45. You can check this answer another way, because 10% of 150 is 15, and three of those make 3 × 15 = 45. Recap: change the percent to a fraction or a decimal, then multiply by the whole.
The mistake people actually make
A common mistake is to treat 25% of 80 as 80 − 25. A percent is a share of the whole amount, not a number you subtract. Remember to write 5% as 0.05, not as 0.5.