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?  Choose the correct choice from the following answers
A) 4.5 hrs	B) 5 hrs	C) 6.5 hrs	D) 7.2 hrs	E) 7 hrs
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 hrs
<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?