Question 253212
We had a similar  problem in the last week or so.
This version starts with a full tank and drains faster than it fills.
t/30-t/45=1
Let's think about this.
If the fill pipe were closed it would take 30 minutes
So after 30 minutes the original water is gone.
In the meantime, it has filled up for 30 minutes. Let's deal with a complete refill.
It can fill in 45 minutes but has only has 30 minutes so it refilled 2/3
So after 45 minutes it will have filled again but the first fill is gone and the drain has had 15 minutes to empty it.
So it emptied half of it.  To empty all of it it needs another 45 minutes.
So it would take 90 minutes to empty the full tank with the fill pipe opened.