Pipe A can fill a tank in 5 hours, pipe B in 10 hours, and pipe C in 30 hours. When all three pipes are opened simultaneously, how long will it take to fill the tank completely?

Explanation

The combined rate of filling the tank per hour is the sum of each pipe's rate: (1/5) + (1/10) + (1/30) = 1/3. Therefore, all three pipes together will fill the tank in 3 hours.