How many 1/2 ft unit cubes can be added to fill a rectangular prism that has a value of 11.25 cubic ft.

We know that

[volume of one cube]=b³
where
b is the length side of the cube
b=1/2 ft
[volume of one cube]=(1/2)³> 1/8 ft³

if  1 cube has a volume of> 1/8 ft³
 x cubes > 11.25 ft³
x=11.25/(1/8)> x=11.25*8> x=90 cubes

the answer is
90 cubes