1. if you change the unit of measure for the side length of a cube from a larger unit to a smaller unit, what happens to the number of the unit cubes that the cube contains?

No matter what units you are exchanging to and from, the number of unit cubes will remain constant. For example: If I had a cube who's side was 10 km long, the number of unit cubes would be the volume of the cube which in this case is 1000 km³. So there would be 1000 unit cubes. Then, if I changed the side from 10 km to 10 in, and find the volume, it would be 1000 in³, meaning there are 1000 unit cubes! So the answer to your question is: The number of unit cubes remains constant

She should use 12/3 to measure