The statement "unit cubes with side lengths of 1/2 foot are added to fill the prism with no space remaining" is useless information so you can disregard that. All you need to know to calculate the volume is the length, width, and height.
Volume = length x width x height Volume = (5/2) x 3 x 11/2 Volume = 15/2 x 11/2 Volume = 165/4 Volume = 41.25 ft cubed
