Pipe A can fill a tank in 16 minutes, while pipe B can drain it in 24 minutes. If both pipes are opened simultaneously, after how many minutes should pipe B be turned off so that the tank is completely filled in 30 minutes?

Explanation

Let the time after which pipe B is closed be x minutes. The amount filled by pipe A in 30 minutes is 30/16, and the amount drained by pipe B in x minutes is x/24. The net filling must equal 1 tank volume: (30/16) - (x/24) = 1. Solving for x gives x = (30/16 - 1) * 24 = (14/16) * 24 = 21 minutes, which is not listed among the options. Therefore, the correct choice is 'None of the above'.