Two pipes A and B can fill a cistern in 20 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?	Select:
A) 19 1/7 min	B) 15 1/7 min	C) 17 1/7 min	D) 7 1/7 min	E) 7 2/7 min
1/20 + 1/30 - 1/40 = 7/120   120/7 = 17 1/7   ANSWER:17 1/7 min
<result>C</result>
Two pipes A and B can fill a cistern in 20 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?