Two pipes, A and B, can fill a tank individually in 4 hours and 5 hours respectively. If they are operated alternately for one hour each, how long will it take to fill the tank completely?

Explanation

Pipe A fills 1/4 of the tank in one hour, and pipe B fills 1/5 of the tank in one hour. Operating alternately for two hours, they fill (1/4 + 1/5) = 9/20 of the tank. To fill the entire tank, it takes 2 full cycles (4 hours) to fill 9/10 of the tank. The remaining 1/10 is filled by pipe A in (1/4) hour, which is 0.4 hours or 24 minutes. Therefore, total time required is 4 hours 24 minutes.