Percentages, made simple.
Percent means out of one hundred. A percentage expresses a quantity relative to a whole, making different-sized groups easier to compare. If 20 out of 25 pupils bring a water bottle, that is 80%. This does not mean there are 80 pupils; it means the same proportion as 80 out of 100.
Connect the three forms
A percentage, a fraction and a decimal can name the same value. For example, 25% = 25/100 = 1/4 = 0.25. Divide a percentage by 100 to write it as a decimal.
Find a percentage of a number
Multiply the whole by the percentage and divide by 100. To find 15% of 200, calculate 200 × 15 ÷ 100 = 30. Always identify which number represents the whole before starting.
Work from a part to a percentage
Divide the part by the whole, then multiply by 100. A score of 18 out of 24 is 18 ÷ 24 × 100 = 75%. This calculation requires a nonzero whole.
Use easy percentages
10% is one tenth, 50% is one half and 25% is one quarter. To find 15%, you can add 10% and 5%. These mental methods are useful checks on a written calculation.
A library has 240 books. If 35% are storybooks, how many storybooks are there?
- The whole is 240 books.
- Find 30%: 240 × 30 ÷ 100 = 72.
- Find 5%: 240 × 5 ÷ 100 = 12.
- Add the two parts: 72 + 12 = 84.
84 storybooks
Take it with you: Percentages describe a share of a whole. Name the whole first, then calculate.