Two pipes can fill a tank in 12 and 15 hrs and third pipe can empty it in 6 hrs. The first two pipes are kept open for 5 hrs in the beginning and after that third pipe also opened. After starting all three pipes, how much time the cistern took to empty.

45 hrs

Tank filled in first five min by A + B = 9 × 5 = 45 ltr

Efficiency of A, B and C = 5 + 4 - 10 = -1

Therefore, pipe C can empty it in = \(\frac{45}{1}\) = 45 hrs.