A significant difficulty with mental calculation is the load that it places on working memory.
One way to ease that load is to turn numbers that are difficult to add and subtract into 'friendly numbers' that are easy to work with.
In the addition process this usually involves:
- rounding numbers up to the nearest multiple of 10 or 100
- subtracting the extras that were added at the end.
In the subtraction process this usually involves subtracting too much and completing the calculation by putting some back.