Aright rectangular prism has a length of 2 1/2 feet, a width of 3 feet, and a height of 1 1/2 feet. unit cubes with side lengths of 1/2 foot are added to completely fill the prism with no space remaining. what is the volume, in cubic feet, of the right rectangular prism

As I think it is easy to answer the volume of prism I will also calculate no of small prism that can fit inside the larger one  
so volume = length * width * height
 volume of larger prism
 vp=45/4 ft^3 =11.25
 volume of small cubes
 vs=1/2*1/2*1/2
 vs=0.125 ft^3
 now numbers of cube that can fit
 n=vp/vs
 n= 11.25/.125
 n=90
 so 90 smaller cubes will completely fill in the prism