How does a function model the relationship between two quantities?

It shows the relationship that exists between the 2 variables or numbers, the operations that are done to the x variable shows the major types of relationships that exist Between x and y.

As for all simple functions y = f(x).

No he does not because he can only cover 12 square feet from 15 square feet which he will still need 3 more. this is because if 3×5=15 square feet and 15-12=3 which thats the answer