A cistern can 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 ? Choices.
A) 7 hours
B) 7.1 hours
C) 7.2 hours
D) 7.3 hours
E) 7.4 hours
Explanation:   When we have question like one is filling the tank and other is empting it, then we subtraction as,   Filled in 1 hour = 1/4   Empties in 1 hour = 1/9   Net filled in 1 hour = 1/4 - 1/9   = 5/36   So cistern will be filled in 36/5 hours i.e. 7.2 hours   ANSWER IS 7.2 hours
<result>C</result>
A cistern can 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 ?