The final cost of a book store purchase with a $5 coupon in a state that charges 8% sales tax is given by the expression 1.08(x-5). the variable x represents the amount of the purchase before the sales tax and the coupon is applied. what does (x-5) represent? a)the rate at which the total cost increases b) the total charge after tax c) the cost before applying the coupon d) the total charge before applying sales tax