I sold a book at a profit of 10%. Had I sold it for $90 more, 15% would have been gained. Find the cost price? Select:
A) $ 1800  B) $ 2500  C) $ 3000  D) $ 3120  E) $ 1540

Rationale: "115 % of cost - 110 % of cost = $ 90 5 % of cost = $ 90 cost = 90 * 100 / 5 = $ 1800 answer is a"
Answer: A