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?
[a] 17 1/4 min
[b] 28 1/7 min
[c] 17 1/7 min
[d] 17 1/8 min
[e] 17 5/7 min