Pipes A and B can fill a tank in 20 h and 30 h, respectively and pipe C can empty the full tank in 40 h. If all the pipes are opened together, how much time will be needed to make the tank full?
Correct Answer: 171/7 h
Explanation:
Net part filed in 1 h = (1/20 + 1/30 - 1/40) = 7/120
Hence, the tank will be full in 120/7 = 171/7 h
