I sold a book at a profit of 10%. Had I sold it for $150 more, 15% would have been gained. Find the cost price? Choose one:
A) $ 2000
B) $ 2500
C) $ 3000
D) $ 3120
E) $ 1540

Rationale: "115 % of cost - 110 % of cost = $ 150 5 % of cost = $ 150 cost = 150 * 100 / 5 = $ 3000 answer is c"
Answer: C