A pipe can fill a tank with water in 3 hours. Due to leakage in bottom, it takes 31/2 hours to fill it. In what time the leak will empty the completely filled tank?

Answer: Option B

Let the total capacity of tank is 21 units
A's efficiency is 7 units/hr
A's efficiency after leakage 6 units/hr
leakage efficiency = 7 - 6 = 1 unit/hr
leakage will empty the filled tank
$$\frac{\text{Total Capacity}}{\text{Efficiency }}=\frac{21}{1}$$
= 21hours