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?

Explanation:-

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