How to add and subtract in your head

Rounding numbers

Rounding numbers to the nearest 1, 10 or 100 can help with lots of mental maths problems.

  • To add 9 to another number, add 10 and then subtract 1: 36 + 9 = 36 + 10 - 1 = 45
  • To add 18 to another number, add 20 and then subtract 2: 48 + 18 = 48 + 20 - 2 = 66
  • To add 97 to another number, add 100 and then subtract 3: 439 + 97 = 439 + 100 - 3 = 536
  • To add 0.9 to another number, add 1 and then subtract 0.1: 3.7 + 0.9 = 3.7 + 1 - 0.1 = 4.6
36 + 9 = 45

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