GIVEN : Three pipes A, B and C can fill a tank in 10 hours. After working together for 2 hours, C is closed and A, and B can fill the leftover capacity of tank in 6 hours. CALCULATION : Let the capacity of tank = 30 liters Given that tank is filled in 10 hours by all the three pipes Efficiency of all the three pipes working together = 30/10 = 3 liters /hour Pipe C is closed after 2 hours. Capacity filled in 2 hours by the three pipes = 3 × 2 = 6 liters Capacity left = 30 - 6 = 24 liters Now this 24 liters is filled by pipe A and B in 6 hours Efficiency of pipe A + B = 24/6 = 4 liters/hour Efficiency of Pipe C = total efficiency (A + B + C) - Efficiency of (A + B) = 3 - 4 = -1 liters/hour Since the efficiency is in negative hence C is an outlet pipe and it can empty the tank completely in = 30/(-1) = 30 hours (- sign neglected in time) ∴ Pipe C is an outlet pipe and empty the tank in 30 hours