Hello!
First of all we need to find the volume of the eraser. To do so we just multiply all of the sides.
2·1·0.25=0.5 in
Now we find the volume of the whole box.
5·8·2=80 in.
Now we subtract the eraser volume from our box.
80-0.5=79.5
The volume of the box not taken up by the eraser is 79.5 inches.
I hope this helps!