A cistern is filled by tap A in 4 hours where as it can be emptied by tap B in 9 hours. If both the taps are opened simultaneously, in how much time will the cistern be filled ?
Answer: C
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