A tank is filled by 3 pipes A,B,C in 4 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 one:
A) 25 hr
B) 35 hr
C) 40 hr
D) 20 hr
E) 28 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 / 4 7 / x = 1 / 4 x = 28 hours answer is e"
Answer: E