A tank has a leak which would empty the completely filled tank in 10 hours. If the tank is full of water and a tap is opened which admits 4 litres of water per minutes in the tank, the leak takes 15 hours to empty the tank. how many litres does the tank hold?

Answer: Option D

Let the capacity of the tank = q litres
According to the question,
Quantity of water emptied by the leak in 1 hour = $$\frac{\text{q}}{10}$$ litres
Qunatity of water filled by the tap in 1 hour = 240 litres
According to the question,
$$\begin{array}{rl}& \Rightarrow \frac{\text{q}}{10}-\frac{\text{q}}{15}=240\\ & \Rightarrow \frac{3\text{q}-2\text{q}}{30}=240\\ & \Rightarrow \frac{9}{30}=240\\ & \Rightarrow \text{q}=240\times 30\\ & \Rightarrow \text{q}=7200\text{litres}\end{array}$$