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?

Answer: Option C

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.
$$\begin{array}{rl}& \therefore \frac{1}{x}+\frac{2}{x}+\frac{4}{x}=\frac{1}{5}\\ & \Rightarrow \frac{7}{x}=\frac{1}{5}\\ & \Rightarrow x=35\text{hours}\end{array}$$