A tank is filled in 8 hours by three pipes A, B and C. The pipe C is twice as fast as B and B is twice as fast as A. How much time will pipe A alone take to fill the tank?

Let
Pipe A can fill the tank in x hours. Then,
Pipe B can fill the tank in x/2 hours,
Pipe C can fill the tank in x/4 hours.

Part filled by pipe A in 1 hour = 1/x
Part filled by pipe B in 1 hour = 2/x
Part filled by pipe C in 1 hour = 4/x

Now
1/x + 2/x + 4/x = 1/8
(1 + 2 + 4)/x = 1/8
7/x = 1/8
x = 56 hours