I am assuming that the cost of an oil change already includes the mark-up.
So, 18 is 175%. 18 ÷ 1.75 = 10.29 is the original cost.
10.29 * 3/4 = 30.87/4 = 7.71 mark-up
10.29 + 7.71 = 18
Or if 18 is the basis of the mark-up, then,
18 * 3/4 = 54/4 = 13.50 mark-up
18 + 13.50 = 31.50 new cost of an oil change