A pipe 'A' can fill a tank in 24 minutes, and another pipe 'B' can fill it in 15 minutes, but the third pipe 'C' can empty it in 30 minutes. Pipe A is opened for 4 minutes and pipe B is opened for 6 minutes and then it is closed. Find out for how long pipe 'C' will empty the tank?

In 4 minutes, pipe A fills (1/24) × 4 = 1/6 of the tank. In 6 minutes, pipe B fills (1/15) × 6 = 2/5 of the tank. Total filled = 1/6 + 2/5 = 17/30 of the tank. Remaining to empty = 1 - 17/30 = 13/30 of the tank. Pipe C empties at rate 1/30 per minute. Time needed = (13/30) ÷ (1/30) = 13 minutes. However, this doesn't match any option. Wait - rechecking: 1/6 + 2/5 = (5+12)/30 = 17/30. Empty = 13/30. Pipe C rate = 1/30 per minute. Time = 13 minutes. But 13 is not an option. Let me reconsider: Perhaps the question asks how long pipe C needs to run to make the tank empty? If tank is 17/30 full, pipe C needs to empty 17/30 at rate 1/30, giving 17 minutes. Option D is correct.