Answer:
1/625, or 0.0016 is the answer.
Here is your answer

Since the door will be in shape of a rectangle where,
length= 80 inches
breadth= 72 inches
So,
widht of largest mattress that can fit diagonally is diagonal of the door.
i.e.





HOPE IT IS USEFUL
You would have to say how much students are in the school
It should take about 3.25 minutes