Cistern may 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 cistern will get filled ? Pick one.
A) 8 hrs
B) 8.5 hrs
C) 7 hrs
D) 7.2 hrs
E) 9 hrs
filled in 1 hr = 1/4   empties in 1 hr = 1/9   net filled in 1 hr = 1/4-1/9   ==&gt;filled in 36/5 ==&gt; 7.2 hrs   ANSWER 7.2 hrs
<result>D</result>
Cistern may 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 cistern will get filled ?