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?
Answers:
A) 7.5	B) 7.4	C) 7.9	D) 7.2	E) 7.1
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.Answer: 7.2
<result>D</result>
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?