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?	Choose one:
A) 17 1/9 min  B) 17 1/8 min  C) 17 1/7 min  D) 17 8/7 min  E) 17 3/7 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?