Percentage is expressed in terms of 100.
80% of c means 80/100 * c. Since the result is 20, it means that
80/100 * c = 20
0.8c = 20
c = 20/0.8
c = 25
200% of d means 200/100 * d. Since the result is 18, it means that
200/100 * d = 18
2d = 18
d = 18/2
d = 2