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