A cistern can be filled by a tap in 4 hours and emptied by another tap in 9 hours. If both taps are opened simultaneously, after how much time will the cistern be filled?
A
7 hours
B
7.3 hours
C
7.2 hours
D
None of these
Correct Option:
C
Explanation
Fill rate = 1/4 tank per hour, emptying rate = 1/9 tank per hour. Net rate = 1/4 − 1/9 = 5/36 tank per hour. Time = 36/5 = 7.2 hours.
