Two pipes, A and B, can fill a tank in 15 minutes and 20 minutes respectively. Both pipes are opened simultaneously, but pipe A is closed after 4 minutes. How long will it take in total to fill the tank completely?

Explanation

In 4 minutes, both pipes fill: 4 × (1/15 + 1/20) = 7/15 of the tank. The remaining portion is 1 - 7/15 = 8/15. Pipe B alone fills at a rate of 1/20 per minute. To fill 8/15 of the tank, pipe B needs (8/15) ÷ (1/20) = 10⅔ minutes (10 minutes 40 seconds). Therefore, total time = 4 minutes + 10 minutes 40 seconds = 14 minutes 40 seconds.