1/20 + 1/30 - 1/40 = 7/120
120/7 = 17 1/7
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] 17 1/9
[B] 17 1/0
[C] 17 1/7
[D] 17 1/2
[E] 17 1/3