Question

Matias stacked his cubes to make this rectangular prism.

rectangular prism with grid lines


How many cubes did Matias use?



cubes

Answer 1

The answer is 80

Answer 2

I FOUND YOUR COMPLETE QUESTION IN OTHER SOURCES.
SEE ATTACHED IMAGE.
The problem can be modeled as a pair of matrices:
First matrix:
(4) * (4) = 16
Second matrix:
(16) * (5) = 80
Since the second matrix depends on the first, then the number of cubes is:
N = 80
Answer
80 cubes.