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.

Mental maths - methods for addition and subtraction

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

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?

Mental addition and subtraction

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.

Skills practice

There's more to learn

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

KS3 Maths
BBC Bitesize Daily
Play Nightfall
game
11 - 14 Maths
KS3 English
A Brief History of Mathematics