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?
Options:
A) 19 1/7 min B) 15 1/7 min C) 17 1/7 min D) 7 1/7 min E) 9 1/7 min
Explanation:   1/20 + 1/30 - 1/40 = 7/120   120/7 = 17 1/7   Answer: Option 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?