Two pipes can fill a tank in 10hours 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?

A. 7 hrs

B. 7 hrs 30 min

C. 5 hrs

D. 5 hrs 30 min

Correct Answer :

B. 7 hrs 30 min

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