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 1/3
C) 17 1/7
D) 17 1/2
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?