POTW76

Two pipes can be used to fill up a swimming pool. The first can fill the pool in three hours, and the second can fill the pool in four hours. There is also a drain that can empty the pool in six hours. Both pipes were being used to fill the pool. After an hour, a careless maintenance man accidentally opened the drain. How long more will it take for the pool to fill?

This is problem 25 from the 1995 SMO Junior Round 1.