Two pipes A and B can fill a cistern in 10 and 30 minutes respectively, and a third pipe C can empty it in 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?  Pick.
A) 19 1/7 min	B) 15 1/7 min	C) 17 1/7 min	D) 9.2 min	E) 7 2/7 min
1/10 + 1/30 - 1/40 = 13/120   120/13 = 9.2   ANSWER:9.2 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 40 minutes. How long will it take to fill the cistern if all the three are opened at the same time?