I sold a book at a profit of 10%. Had I sold it for $120 more, 15% would have been gained. Find the cost price? Pick.
A) $ 2000	B) $ 2500	C) $ 2400	D) $ 3120	E) $ 1540

Rationale: "115 % of cost - 110 % of cost = $ 120 5 % of cost = $ 120 cost = 120 * 100 / 5 = $ 2400 answer is c"
Answer: C