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?

The answer is 6. The inlet pipe is two times faster than the drainpipe. Thus, the inlet pipe will need double the time to fill the pool to capacity.