Compute 3/4 of 20.
20 ÷ 4 = 5
I divide by the bottom number: 20 shared into 4 equal parts, each part is worth 5. A quarter of 20 is 5.
5 × 3 = 15
I multiply by the top number: I want 3 quarters, so 3 parts of 5, which is 15.
Result: 3/4 of 20 = 15
Back to the path
¼
Fractions of a quantity
Level 2 · Ages 8–11
Quick recall· Let's wake up what you already know before we start.
Recall: a pizza is cut into 4 equal parts and you take 1. What fraction of the pizza did you take?
The whole lesson
Read it again any time, including while you work through the exercises.
You now know that a fraction is a sharing into equal parts. Good news: you can also take a fraction of a quantity. Half of 18 marbles, a quarter of 20 euros, three quarters of an hour: these are very useful calculations in real life.
The method takes two moves. To compute 3/4 of 20, I start with the denominator: I divide 20 by 4, which gives 5. Each quarter of 20 is worth 5. Then I use the numerator: I want 3 of these quarters, so I multiply 5 by 3, which gives 15. Three quarters of 20 is 15.
Remember the magic sentence: I divide by the bottom number, I multiply by the top number. Divide first, multiply after: the numbers stay small and the calculation stays easy.
To check, you can always picture objects. 20 sweets shared into 4 equal bags: 5 sweets per bag. You take 3 bags: 15 sweets. The drawing and the calculation tell the same story.
Your turn: in this lesson you will compute halves, thirds, quarters and even fifths of all kinds of quantities.
The worked example, step by step
Compute 2/3 of 18.
18 ÷ 3 = 6
I divide by 3: one third of 18 is 6.
6 × 2 = 12
I multiply by 2: two thirds is two parts of 6, so 12.
Result: 2/3 of 18 = 12