# Using mental strategies to add and subtract

## Home learning focus

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.

## 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 subtract97 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

• 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

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.