If A is x% more than B, this means that A = (1+x/100)B. In the same way, if A is x% less than B, this means that A = (1- x/100)B.

So if the price of the new car is N, and the price of the old car is O, then N = (1+25/100)O. You are asked to find x such that O is x% less than N, or O = (1- x/100)N. Substitute for N to get O = (1-x/100)(1+25/100)O or 1 = (1-x/100)(1+25/100). Now solve for x.