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? Choose the correct option:
A) 7 hrs 30 min	B) 7 hrs 45 min	C) 8 hrs 30 min	D) 8 hrs 45 min	E) 8 hrs 55 min
1/10+1/12-1/20=2/15   so 15/2 is equal to 7:30   7 hrs 30 minNSWER:7 hrs 30 min
<result>A</result>
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?