A can fill a tank in 15 hours. Due to a leak, it takes 20 hours to fill. If the tank is full, how much time will the leak take to empty it?

Correct option is A

Ans. (a)
Sol.
Let the capacity of the tank = 1 unit (for simplicity)
Filling rate of A = 1/15 per hour
Due to the leak, the effective filling rate = 1/20 per hour
Let the leak’s emptying rate be x (per hour). Then:
Effective rate = Filling rate – Leak rate
⇒ 1/20 = 1/15 – x
⇒ x = 1/15 – 1/20
Find LCM of 15 and 20 = 60
⇒ x = (4 – 3)/60 = 1/60
So, the leak alone will empty the full tank in 60 hours.