Two taps A and B can fill a tank in 5 hours and 20 hours respectively. If both the taps are open then due to a leakage, it took 30 minutes more to fill the tank. If the tank is full, how long will it take for the leakage alone to empty the tank ?

A.  8 hrs
B.  9 hrs
C.  18 hrs
D.  36 hrs
Answer: D

Explanation:

Part filled by (A + B) in 1 hour = (1/5 + 1/20)
                                               = 1/4.
So, A and B together can fill the tank in 4 hours.
Work done by the leak in 1 hour    = (1/4 - 2/9)
                                                     = 1 / 36.
Therefore, Leak will empty the tank in 36 hrs.