You are offering 16.4% for customers who purchase the 6-coupon book for the oil changes. This problem can be solved by doing a simple percentage formula of mathematic. The customer had to pay $80 only for six oil changes which cost $95.7 ($15.95 * 6) at the original price and the difference between the coupon price and the original price is the discount (($95.7-$80)/$95.7 *100%).
