A tank is filled by 3 pipes A,B,C in 8 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 answer
A) 25 hr B) 35 hr C) 56 hr D) 20 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 / 8 7 / x = 1 / 8 x = 56 hours answer is c"
Answer: C