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. 5 hrs
C. 6.5 hrs
D. 7.2 hrs

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.