Answer: So, you have 16 trays of hot food, and each box can have a maximum of 5 trays.
a) if each box can have 5 trays then:
1 box can delivery 5 trays, that is not enough.
2 boxes can delivery 10 trays, that is not enough
3 boxes can delivery 15 trays, almost, but not enough
4 boxes can delivery 20 trays, this is it!
You also can see it in the next way: 16 trays/5 trays per box = 3.2 boxes.
But you can't use a 0.2 of a box, so here we need to round upwards!
3.2 rounded upwards is 4.
b) If you have 4 boxes, then you have 20 spaces for trays.
if each box is filled, you will put the first 15 trays in 3 boxes and fill them, and one leftover tray will go in the fourth box.
so this last box has one of five trays, this means that has 4 slots empty.
then the empty fraction is the empty slots divided by the total number of slots, this is: 4/5.