A tank is filled in 5 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?

Suppose pipe A alone takes x hours to fill the tank.
Then, pipes B and C will take $$\frac{x}{2}$$ and $$\frac{x}{4}$$ hours respectively to fill the tank.
$$\eqalign{ & \therefore \frac{1}{x} + \frac{2}{x} + \frac{4}{x} = \frac{1}{5} \cr & \Rightarrow \frac{7}{x} = \frac{1}{5} \cr & \Rightarrow x = 35\,{\text{hours}} \cr} $$