Children always seem very reluctant to check the answers to calculations that they have made. This sometimes results in bizarre answers, often when using a calculator.
Here is a simple subtraction worksheet but it also asks for each subtraction to be checked by carrying an addition.
For example: 40 – 17 = 23 can be checked by adding 17 and 23 which equals 40.
The most comon error that children make with this is to say that 40 -17 = 33.