Are you good at doubling numbers? This can help you add numbers that are close to each other.

The trick is to double a number that you know and then add on or take away the difference.

If you know that 10 + 10 = 20 then this can help you work out 10 + 11.

10 + 11 is the same as 10 + 10 + 1 which makes 21.

Example

9 + 12 = ?

9 and 12 are both close to 10.

10 + 10 = 20

9 is 1 less than 10, so take this away.

20 – 1 = 19

That leaves the 12. It is 2 more than 10 so you need to add this on.

19 + 2 = 21

You’ve just worked out that 9 + 12 = 21.