IGCSE Maths / Fractions, decimals and rounding

A fraction of a quantity

Edexcel International GCSE Maths (4MA1), chapter 1: Fractions, decimals and rounding, specification point 1.2d. Free lesson.

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To find a fraction of an amount, divide by the denominator and multiply by the numerator. To write one number as a fraction of another, put the whole amount on the bottom and simplify.

How to find a fraction of an amount

In maths, of means multiply. You can multiply the amount by the fraction directly, or divide first to find one equal part and then multiply by the number of parts you need. Dividing first usually keeps the arithmetic simpler. The denominator is the bottom number. It tells you how many equal parts the whole is split into. The numerator is the top number. It tells you how many parts to take.

How to work out 3/5 of 45

First find one fifth of 45 by dividing by 5. Then take three of those parts. You can check the answer by noticing that 3/5 is a little more than half, so the answer should be a little more than half of 45.

How to write one number as a fraction of another

When you describe one number as a fraction of another, the whole amount goes on the bottom because it is the amount you are comparing against. The part being counted goes on top. Then simplify by dividing the top and bottom by the same factor. For 12 students out of a class of 30, write 12/30, not 30/12. A fraction bigger than 1 cannot describe a part that is smaller than the whole.

Check the order and simplify

The denominator tells you how many equal parts make the whole, so divide by it first. The numerator tells you how many of those parts to take, so multiply by it. Simplify fractions before writing your final answer, as an unsimplified answer can lose the final mark. For 5/8 of 72, divide 72 by 8, then multiply by 5. For 18 as a fraction of 48, put 18 over 48 and simplify.

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