# Combining ratios

I can combine ratios.

## Lesson details

### Key learning points

- There may be a relationship between quantities A and B and a relationship between quantities B and C
- Therefore there is a relationship between A and C
- If A : B and B : C then A : B : C and A : C
- When the common quantity is not the same in each ratio, equivalent ratios can be used
- For example, if A:B and 2B:5C then 2A:2B:5C and 2A:5C

### Common misconception

Combining ratios without using equivalent ratios to equate the common part.

Pupils may need to spend extra time finding equivalent ratios and looking at when it is ok to combine ratios or not.

### Keywords

Proportion - A part to whole (sometimes part to part) comparison. If two things are proportional then the ratio of part to whole is maintained and the multiplicative relationship between parts is also maintained.

Ratio - A ratio shows the relative sizes of 2 or more values and allows you to compare a part with another part in a whole.

LCM - LCM is an abbreviation for lowest common multiple.

Lowest common multiple - The lowest common multiple is the lowest number that is a multiple of two or more numbers.

### Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

## Video

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## Starter quiz

### 6 Questions

16 : 12

14 : 12

16 : 40

12 : 32

12 : 22