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Mental Addition & Subtraction

Maths: Year 5 & 6 National Curriculum, topic 1.2.3

Mental Addition and Subtraction

You need to think about our place value rules when adding and subtracting mentally.

4,372 + 6

4,372 + 6

  • You look at the 'ones' column because that is the column you are adding to.
  • I know that 2 + 6 = 8
  • So 4,372 + 6 = 4,378
Carrying over

Carrying over

  • Sometimes you need to carry over a value to the next column.
Carrying over example

Carrying over example

  • E.g. 4372 + 9
  • You look at the 'ones' column because that is the column you are adding to.
  • I know that 2 + 9 = 11
  • So I know that my 'ones' column will be 1 and I need to carry one ten to the 'tens' column.
  • So 4,372 + 9 = 4,381
A different method

A different method

  • There are loads of different ways to do mental addition.
  • I'm trying to do the same calculation as the previous example (4,372 + 9).
  • Instead of adding 9, I can add 10 because that's easier!
  • I know 4,372 + 10 = 4,382
  • But I've added 1 too many, so I need to take 1 away.
  • 4,382 − 1 = 4,381
  • So 4,372 + 9 = 4,381

Adding Double/Triple/Quadruple Digit Numbers in Your Head

You can also add more than one digit to a number. You can add more than one digit to a number by splitting the number into its place values.

Example

Example

  • I want to work out 4,372 + 25.
  • I can split 25 into its place values.
  • 25 = 20 + 5
First, add the 'tens'

First, add the 'tens'

  • I have split 25 into 20 + 5.
  • First, add 20 to 4,372
  • 4,372 + 20 = 4,392
Then, add the 'ones'

Then, add the 'ones'

  • Then, add 5 to 4,392
  • 4,392 + 5 = 4,397
  • So you know that 4,372 + 25 = 4,397
Adding larger numbers

Adding larger numbers

  • You can use the same process to add larger numbers in your head.
  • E.g. 4,372 + 214
  • Start by splitting 214 into 200 + 10 + 4.
Adding 214

Adding 214

  • Then add 200 to 4,372.
  • 4,372 + 200 = 4,572
  • Then add 10 to 4,572.
  • 4,572 + 10 = 4,582
  • Then add 4 to 4,582.
  • 4,582 + 4 = 4,586

Test yourself on Mental Addition & Subtraction

  • Which of these calculations might you do to make calculating 6,786 + 9 easier?

  • Which of these calculations might you do to make calculating 488 + 9 easier?

  • How would you split 227 into its place values to make addition easier?

  • How would you split 124 into its place values to make addition easier?

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