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Using Ratios

Maths: Year 5 & 6 National Curriculum, topic 6.1.1

Ratios

Ratios describe how numbers change in relation to each other. For example, if I double the amount of flour in a recipe, then I will need to double all of the other ingredients as well.

Ratios in recipes

Ratios in recipes

  • A recipe says that for every 1 g of sugar, you need 5 g of flour.
  • The ratio of sugar to flour is 1 : 5.
Using ratios

Using ratios

  • If I use 10 g of sugar, how much flour will I need?
  • I used 10× as much sugar.
  • So I need 10× as much flour.
  • You must do the same calculation to both sides of the ratio.
  • 10 g of sugar needs 50 g of flour.
Another example

Another example

  • A class has 3 girls for every 2 boys.
  • Our ratio of girls to boys would be 3 : 2.
Example Question

Example Question

  • If there were 8 boys in a class, how many girls would there be?
  • You have multiplied the number of boys in the ratio by 4 to make 8 (2 × 4 = 8).
Multiply both sides by 4

Multiply both sides by 4

  • You need to multiply both sides by the same amount to keep the ratio the same.
  • You need to multiply 3 by 4 to find out how many girls there are in the class (3 × 4 = 12).
Final answer

Final answer

  • There are 12 girls and 8 boys in the class.

Test yourself on Using Ratios

  • Which of these is in the same ratio as 6 : 9?

  • Which of these is in the same ratio as 3 : 7?

Practise this topic for free