A cistern can be filled by a tap in 3 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 choices: A) 2.9 hrs B) 4.5 hrs C) 2.9 hrs D) 7.2 hrs E) 8.6 hrs
Net part filled in 1 hour = (1/3 - 1/9)   = 2/9   The cistern will be filled in 9/2 hrs i.e., 4.5 hrs.   Answer:4.5 hrs
<result>B</result>
A cistern can be filled by a tap in 3 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?