ok so
1 find volume of 1 cube
2. find total volume of rectangular prism
1/2 m cubes
volume=side^3
side=1/2m (I guess since that was what was given)
v=1/2^3=1/8
so
volume of 1 cube times number of cubes=total volume
1/8m^3 times number of cubes=5m^3
multiply both siides by 8/1 m^3 to clea fraction (since 8/1 times 1/8=8/8=1)
number of cubes=40
needs 40 cubes