# Using mental strategies to add and subtract

## Home learning focus

In this lesson, you will learn how to add and subtract numbers in your head.

This lesson includes:

- a video
- a learning summary
- three activities

# Learn

You won't always have access to a calculator, so its useful to know how to add and subtract numbers in your head. Two useful strategies are **rounding numbers** and **breaking down numbers.**

Watch this video from BBC Bitesize for more information.

## Rounding numbers

Rounding numbers to the nearest 1, 10 or 100 can help with lots of mental maths problems. For example:

- To add
**9**to another number, add**10**and then subtract**1:**- 36 + 9 = ?
- 36 + 10 = 46
- 46 - 1 = 45
- So,
*36 + 9 = 45*

- To subtract
**9**from another number, subtract**10**and then add**1:**- 36 - 9 = ?
- 36 - 10 = 26
- 26 + 1 = 27
- So,
*36 - 9 = 27*

- To add
**97**to another number, add**100**and then subtract**3:**- 439 + 97 = ?
- 439 + 100 = 539
- 539 - 3 = 536
- So,
*439 + 97 = 536*

- To subtract
**97**from another number, subtract**100**and then add**3:**- 334 - 97 = ?
- 334 - 100 = 234
- 234 + 3 = 237
- So,
*334 - 97 = 237*

- To add
**0.9**to another number, add**1**and then subtract**0.1:**- 3.7 + 0.9 = ?
- 3.7 + 1 = 4.7
- 4.7 - 0.1 = 4.6
- So,
*3.7 + 0.9 = 4.6*

- To subtract
**0.9**from another number, subtract**1**and then add**0.1:**- 4.8 - 0.9 = ?
- 4.8 - 1 = 3.8
- 3.8 + 0.1 = 3.9
- So,
*4.8 - 0.9 = 3.9*

## Breaking down numbers

Breaking down numbers can help you add or subtract in your head. For example: find the difference between 38 and 63.

- Count on from 38 to 40 and keep that 2 in your head.
- Now count on from 40 to 60, which is 20
- Finally count from 60 to 63 which is 3
- Add them together: 2 + 20 + 3 = 25
- So the difference between 38 and 63 is
*25*

# Practise

## Activity 1

**Fill in the gaps**

Have a go at these addition and subtraction questions using the mental strategies covered in this lesson.

## Activity 2

**Mental addition and subtraction**

Work through these interactive questions from My Maths, Oxford University Press. Can you get all the way to the end?

## Activity 3

**Worksheet: Skills practice**

Have a go at this worksheet from Hodder Education.

You can print it out or write your answers on a piece of paper.

# There's more to learn

Have a look at these other resources around the BBC and the web.