A cistern can be filled by a tap in 7 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) 17.2 hrs	E) 31.5 hrs
Net part filled in 1 hour= 1/7 -1/9= 2/63   Therefore the cistern will be filled in 63/2 hours or 31.5 hours.   ANSW31.5 hrsR:31.5 hrs
<result>E</result>
A cistern can be filled by a tap in 7 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 ?