A water tank is filled by one pump in 6 hours and is emptied by another pump in 12 hours. If both pumps are operating, how long will it take to fill the tank?

Question

anonymous · Answer

Pump 'A' can fill 1/6 of the tank in one hour. 1/6 is the same as 2/12
Pump 'B' can drain 1/12 of the tank in one hour.
If both pumps are running at the same time, then in one hour 2/12 get filled and 1/12 gets drained, and the net effect is 1/12 getting filled in that hour.
If the tank is initially empty and both pumps start at the same time, then the tank will fill in 12 hours.
 It takes six hours if pump B is left off.

anonymous · Answer

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