That is very simple, you deduct the actual payment from the basic earnings and divide it by the basic earning.
In this case for example, it becomes:
100 - 80/ 100
= 20/100
= 0.2 or 20% is the answer
In this case for example, it becomes:
100 - 80/ 100
= 20/100
= 0.2 or 20% is the answer