Answer:
The discount rate operated by the coupon is 20%.
Explanation:
Since the pair of shoes normally sells for $ 90, but through the application of a discount coupon it sells for $ 72, to determine the discount percentage made by this coupon it is necessary to perform the following calculation: 
90 = 100 
72 = X 
(72 x 100) / 90 = X 
7,200 = 90 = X 
80 = X 
 100 - 80 = 20 
Thus, the discount operated by the coupon is 20%.