a rectangular prism with a volumen of 10 cubic units is filled with cubes with side lengths of 1/2 unit

40

I assumed that you wanted to know how much little cubes are in the big cubes ... so ... first you need to find out the volume of the small cube : 1/2 x 1/2 = 1/4 ... then divide it with 10, then results with 40! : D