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 option:
A) 4.5 hrs B) 5 hrs C) 6.5 hrs D) 7.2 hrs E) None
Solution   Net part filled in 1 hour	= (1/4 - 1/9)   = 5/36.   Therefore,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 ?