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