A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9hours. 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) 6hr
B) 5.6hr
C) 9.5hr
D) 7.2hr
E) 4hr
Net part filled in 1 hour 1/4 - 1/9 = 5/36   The cistern will be filled in 36/5= 
<gadget id="calculator">36/5</gadget>
<output>36/5 = around 7.2</output>
7.2hr   Answer is 7.2hr
<result>D</result>
A cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9hours. If both the taps are opened simultaneously then after how much time will the cistern get filled?