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? Choices:
A) 17 1/6 B) 17 1/9 C) 17 1/7 D) 17 1/5 E) 17 1/1
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?