655 - Math - Problem Solving

Question: An empty swimming pool can be filled to capacity through an inlet pipe in 3 hours, and it can be completely drained by a drainpipe in 6 hours. If both pipes are fully open at the same time, in how many hours will the empty pool be filled to capacity?

1. 4
2. 4.5
3. 5
4. 5.5
5. 6