I Desperately Need Help With Geometry!!!

I am having a hard time with some of my geometry and I need help. I have until next month to submit my work from Khan Academy, but it hard learning it on your own. Right now I am struggling with Volume with cubes of fraction lengths because it is not making any sense to me and when I keep trying, I keep getting it wrong. I keep getting questions like for example: How many cubes with side lengths of 1/2 fill the prism. It is a rectangular prism. The three sides are 2 1/2 cm, 2 cm, and 1 cm. I would really appreciate the help.