Two pipes can fill a tank in 20 and 24 minutes respectively and a waste pipe can empty 3 gallons per minutes. All the three pipes working together can fill the tank in 15 minutes, The capacity of the tank is-

Explanation:-

$$\eqalign{ & {\text{Work done by the waste pipe in 1 minute }} \cr & {\text{ = }}\frac{1}{{15}} - \left( {\frac{1}{{20}} + \frac{1}{{24}}} \right) \cr & = \left( {\frac{1}{{15}} - \frac{{11}}{{120}}} \right) \cr & = - \frac{1}{{40}}\left[ { - \,{\text{Nagetive sign means emptying}}} \right] \cr & \therefore {\text{Volume of }}\frac{1}{{40}}{\text{ part = 3 galons}} \cr & {\text{Volume of whole tank}} \cr & {\text{ = }}\left( {3 \times 40} \right){\text{gallons}} \cr & {\text{ = 120 gallons}}{\text{.}} \cr}$$