A tank is filled by 3 pipes A,B,C in 7 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 ?	Choose the correct choice:
A) 25 hr  B) 35 hr  C) 40 hr  D) 49 hr  E) 50 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 / 7 7 / x = 1 / 7 x = 49 hours answer is d"
Answer: D