Pipes A and B can fill a cistern in 20 hrs and 12 hrs. While C can empty it 15 hrs. Working together in how much time the tank will be filled completely?

15 h

 Let total Capacity of tank = 60 ltr

Efficiency of A, B and C = 3, 5 and -4

Total efficiency of all three pipes = 3 + 5 - 4 = 4 l/hr

Total time taken by all three pipes = \(\frac{60}{4}\) = 15 hours