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.