A pipe can fill a tank in 3 hours. There are two outlet pipes from the tank which can empty it in 7 and 10 hours respectively. If all the three pipes are opened simultaneously, then the tank will be filled in -

Explanation:-

$$\eqalign{ & {\text{Net part filled in 1 hour}} \cr & {\text{ = }}\frac{1}{3} - \left( {\frac{1}{7} - \frac{1}{{10}}} \right) \cr & = \frac{1}{3} - \frac{{17}}{{70}} \cr & = \frac{{19}}{{210}} \cr & \therefore {\text{The tank will be filled in }}\frac{{210}}{{19}}{\text{ hours i}}{\text{.e}}{\text{.}} \cr & {\text{ 11}}\frac{1}{{19}}{\text{hours}} \cr & = 11\,{\text{hours}} \cr}$$