A tank is filled by 3 pipes A,B,C in 6 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 from the following.
A) 25hr  B) 42hr  C) 40hr  D) 20hr  E) 50hr
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 = 1/6   7/x = 1/6   x = 42 hours   Answer is 42hr
<result>B</result>
A tank is filled by 3 pipes A,B,C in 6 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 ?