A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 7 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?
Choose the correct option:
A) 2.9 hrs
B) 8.9 hrs
C) 9.3 hrs
D) 7.2 hrs
E) 8.6 hrs
Net part filled in 1 hour = (1/4 - 1/7)   = 3/28   The cistern will be filled in 28/3 hrs i.e., 9.3 hrs.   Answer:9.3 hrs
<result>C</result>
A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 7 hours. If both the taps are opened simultaneously, then after how much time will the cistern get filled?