Mental Addition & Subtraction

Maths: Year 3 & 4 National Curriculum, topic 1.2.1

Adding Numbers Mentally

We can add numbers from three-digit numbers mentally. This means we work it out in our head without writing anything down.

Adding ones

Adding ones

To work out 463 + 5, look at the ones column:
5 + 3 = 8
463 + 5 = 468

Adding ones

Adding ones

Sometimes, it will affect two columns:
To work out 463 + 8, look at the ones column:
3 + 8 = 11
It is bigger than 10, so both the ones and the tens column will change:
463 + 8 = 471

Adding tens

Adding tens

The same rules apply for 10s, but this time we look at the tens column.
For example, 485 + 10 = 495.
Only the tens column has changed.
But 485 + 30 = 515.
Both the tens and hundred columns have changed.

Adding hundreds

Adding hundreds

To mentally add 100s, we look at the hundreds column.
284 + 400 = 684
Only the hundreds column has changed.
But 284 + 800 = 1084
Both the hundred and thousands columns have changed.

Subtracting Numbers Mentally

We can also subtract numbers from three-digit numbers mentally.

Subtracting ones

Subtracting ones

Mentally subtracting is the same strategy but the inverse (opposite).
For example, 567 - 6 = 561
Only the ones column changes.
But 432 - 6 = 426
The tens and ones column has changed.

Subtracting tens

Subtracting tens

For example, 485 - 70 = 415
Only the tens column changes.
But 954 - 60 = 894
The hundreds and tens column has changed.

Subtracting hundreds

Subtracting hundreds

For example, 631 - 200 = 431
Only the hundreds column changes.

Test yourself on Mental Addition & Subtraction

What is 235 + 4?
What is 560 + 50?
What is 792 + 300?
What is 743 - 70?
What is 135 - 8?

Practise this topic for free