Answer:
Discount percentage is 16.4% for the customers who purchase 6- coupon book .
Step-by-step explanation:
Formula 

As given
 Your station charges $15.95 for an oil change. 
Original charge of the oil change for 6 times = 6 × Original charge for oil change .
                                                                           = 6 × 15.95
                                                                           = $ 95.7
As a promotion you sell six coupons for oil changes for $80. 
Discount = Original charge of the oil change for 6 times - Charge by six coupons for oil changes .
                = $95.7 - $80
                = $ 15.7 
Putting all the values in the above



Discount percentage = 16.4% 
Therefore the discount percentage is 16.4% for the customers who purchase 6- coupon book .