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?

Answer: 6 hours to fill

Step-by-step explanation:

Let's consider what happens in one hour.

If a pool fills in 3 hours, it means that after 1 hour it is

1/3 full.

If a pool empties in 6 hours, it means that in 1 hour it loses

1/6

So if it is filling and draining at the same time, after one hour the difference would be:

1/3-1/6=1/6

The nett result is that the pool fills at a rate of 1/6 per hour.

1:1/6=6 hours to fill.