Question 1055545
Since the number 75 is a composite number, you could pack the cubes in a 5x5x3 configuration.
Since each cube is an inch, the total volume would be {{{5*5*3=75}}}{{{in^3}}}.
So in this configuration, there is no empty space.
If the number of cubes was a prime number, you would need additional empty volume to pack the cubes.