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.