Question

Two pipes can fill a tank in 10 hours and 12 hours respectively while a third, pipe empties the full tank in 20 hours. If all the three pipes operate simultaneously, in how much time will the tank be filled?

Answer 1

Net part filled In 1 hour =(1/10)+(1/12)-(1/20)=(8/60)=(2/15). 

The tank will be full in 15/2 hrs = 7 hrs 30 min.