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? Pick:
A) 17 1/9 B) 17 1/0 C) 17 1/7 D) 17 1/2 E) 17 1/3
1/20 + 1/30 - 1/40 = 7/120   120/7 = 17 1/7   Answer: 17 1/7
<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?