Rounding - subtraction

Rounding is a method for mental subtraction which is useful in many different situations.

Imagine you are shopping and need to work out an amount quickly.

£3.70 - £1.00 = £2.70. Adding 10p gives £2.80.

Have a look at how this subtraction sum can be solved with rounding:
      64 - 17

First, round the 17 up to 20 and work out the sum:
      64 - 20 = 44

As 20 is 3 more than 17, you have taken 3 too many from the total. So you need to add 3:
      44 + 3 = 47

So you can see that:
      64 - 17 = 47

Further reading.
These tables have some more information to help you with subtraction by rounding.

The first shows you a methods and examples for subtracting a number between 11 and 14 from another number.

This table shows methods and examples for subtracting a number between 15 and 19 to another number.