A tank is filled by 3 pipes A,B,C in 3 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) 25hr	B) 35hr	C) 40hr	D) 20hr	E) 21hr
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 = 1/3   7/x = 1/3   x = 21 hours   Answer is 21hr
<result>E</result>
A tank is filled by 3 pipes A,B,C in 3 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 ?