If three identical pipes can fill a tank in 8 hours, how long will it take for only two of these pipes to fill the same tank?

Explanation

Three pipes together fill 1/8 of the tank per hour. Therefore, one pipe fills 1/24 of the tank in an hour. Two pipes will fill 2/24 = 1/12 of the tank each hour, meaning the tank will be completely filled in 12 hours.