How many cubic blocks with a side length of 1/8 inch are needed to fill a rectangular prism with a length, width, and height of 3/4 inch, 1/2 inch, 1/2 inch, respectively
a) 24
b) 48
c) 96
d) 100

Need 6 along the length, 4 along the width, and 4 along the height. 
That would be 96. 
Another way is the volume is (3/16) in^3

Volume of each cube is (1/512) in^3
They fill the space completely.
(96/512)=3/16, when one divides by 32 top and bottom.