If n is the base price then the total cost is 1.075n or 1.075n=total cost.
!00% of the price of the car + 7.5% tax = 107.5% or 1.075 as a decimal.
So when you find 1.075 of the price (n) you are finding the total cost.
Notice how multiplying by the 1 in 1.075 included the cost of the car because 1 x anything is that thing. 1 x n = n