A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?

A. 4.5 hrs
B. 6.5 hrs
C. 5 hrs
D. 7.2 hrs
Answer : D

Explanation :

 Net part filled in 1 hour = (1/4 - 1/9) = 5/36        
The cistern will be filled in    36/5 hrs i.e., 7.2 hrs.