A tank is filled by 3 pipes A,B,C in 10 hours. Pipe C is twice as fast as B and B is twice as fast as A. How much will pipe A alone take to fill the tank ?  Select: A) 20 hr B) 52 hr C) 70 hr D) 66 hr E) 48 hr

Rationale: "suppose pipe a alone take x hours to fill the tank then pipe b and c will take x / 2 and x / 4 hours respectively to fill the tank . 1 / x + 2 / x + 4 / x = 1 / 10 7 / x = 1 / 10 x = 70 hours answer is c"
Answer: C