Matt and mindy each built a rectangular prism that has a length of 5 units, a width of 2 units, and a height of 4 units. matt used cubes that are 1 cm on each side. mindy used cubes that are 1 in. on each side. what is the volume of each prism?

The volume of a prism is just its width times its height times its length.
v=whl

v=2×4×5

v=40

So we know the volume is 40 units. But which units? 

Matt used centimeters as his units, so his rect. prism's volume would be 40 cm³. Mindy's would be 40 in³.