Two pipes A and B can fill a cistern in 10 and 30 minutes respectively, and a third pipe C can empty it in 50 minutes. How long will it take to fill the cistern if all the three are opened at the same time?	Options:	A) 19 1/7 min	B) 15 1/7 min	C) 17 1/7 min	D) 8.8 min	E) 7 2/7 min
1/10 + 1/30 - 1/50 = 17/150   150/17 = 8.8   ANSWER:8.8 min
<result>D</result>
Two pipes A and B can fill a cistern in 10 and 30 minutes respectively, and a third pipe C can empty it in 50 minutes. How long will it take to fill the cistern if all the three are opened at the same time?