Two pipes, A and B, can fill a tank individually in 8 minutes and 24 minutes, respectively. If they are opened alternately for one minute each, how long will it take to fill the tank completely?

Explanation

Pipe A fills 1/8 of the tank per minute, and pipe B fills 1/24 per minute. When opened alternately, in two minutes they fill (1/8 + 1/24) = 1/6 of the tank. Therefore, to fill the entire tank, it takes 6 cycles of 2 minutes each, totaling 12 minutes.