Two pipes A and B can separately fill a cistern in 20 min and 30 min respectively. There is a third pipe in the bottom of the cistern to empty it. If all the three pipes are simultaneously opened, then the cistern is full in 60 min. In how much time, the third pipe alone can empty the cistern? Choose one:
A) 16 min	B) 20 min	C) 25 min	D) 15 min	E) 10 min
Work done by the third pipe in 1 min = 1/60 - (1/20 + 1/30) = - 1/15.   [-ve sign means emptying]   The third pipe alone can empty the cistern in 15 min.   ANSWER:15 min
<result>D</result>
Two pipes A and B can separately fill a cistern in 20 min and 30 min respectively. There is a third pipe in the bottom of the cistern to empty it. If all the three pipes are simultaneously opened, then the cistern is full in 60 min. In how much time, the third pipe alone can empty the cistern?