I sold a book at a profit of 10%. Had I sold it for $100 more, 15% would have been gained. Find the cost price? Choose the correct choice from the following options:
A) $ 2000 B) $ 2500 C) $ 3000 D) $ 3120 E) $ 1540

Rationale: "115 % of cost - 110 % of cost = $ 100 5 % of cost = $ 100 cost = 100 * 100 / 5 = $ 2000 answer is a"
Answer: A