It takes 120 minutes for a hose to fill a swimming pool. If the pool drain is open, it takes 360 minutes to fill the pool. If the pool is full, the hose is off, and drain is open, how many minutes does it take to empty the pool?

I would look at this problem like this:

In those 360 the hose is working, and it's working the same as during the 120 when it filled the pool. Since 360 is three times 120, it means that it filled the pool three times this time. But the pool was only filled once!

This means that twice the pool was drained in those 360 minutes: this makes one draining every 180 minutes. - this is the answer!