A tank is normally filled in 4 hours. But it takes 5 hours to fill, when there is a leak at the bottom of the tank. If the tank is full, in how many hours can it be emptied by the leak?

Correct option is B

Given:

​Pipe A usually fills a tank = 4 hours.
​Due to leakage tank fill in = 5 hours
​Formula Used:
​Work = Time × Efficiency
​Solution:
​LCM of (4, 5) = 20 unit
​Let the total capacity of tank be 20 unit
​ Efficiency of Pipe A = $$\frac{20}{4} = 5\, \text{unit/hrs}$$​​​
​Efficiency of Pipe A with leakage = $$\frac{20}{5} = 4\, \text{unit/hrs}$$​​​
​Efficiency of Leakage = 1 unit/hr
​Leakage empties the tank in = $$\frac{20}1= 20\, \text{hrs}$$