Alright so this is is quite easy, all we need to do is find the volume of the box, and that will give us the number of blocks required to fill the box.
To find the volume of a box we need to multiply the height by the depth by the length:
5x5x5 (or 5^3)
= 125 cubic inches
So 125 blocks would fit in the box.
Hope this helps!
- Z