Sometimes the easiest way to carry out a subtraction question is to do it by counting up, or adding on, from the smaller number to the larger number. This is often the case when the two numbers are close to each other but either side of the hundreds boundary.
For example, 102 - 97 can be done by adding on 3 to 97 to make 100 and then another 2 to make 102. Altogether 5 has been added on. Children may well use their fingers to help do this - there is nothing wrong with that!
Subtract numbers either side of 100