Two pipes P and Q can fill a tank in 20 min and 30 min respectively. If both the pipes are opened simultaneously, after how much time should Q be closed so that the tank is full in 16 minutes?

Pipe P fills at 1/20 tank per minute and Q at 1/30 tank per minute. Let Q be closed after t minutes. P works for all 16 minutes, contributing 16/20 = 4/5 of the tank. Q works for t minutes, contributing t/30. The equation is 4/5 + t/30 = 1, giving t = 6 minutes. If Q stayed open the full 16 minutes, they would overfill the tank (16/12 > 1).