The amount of books Matt had at the beginning (5) is a constant that does not change, thus nothing will be multiplied by that number. This eliminates options A and C, which both demonstrate 5 as being multiplied by the amount, x, of weeks.
As for the other two equations, which one makes sense for this problem, which models an increase of books over time? The equation with addition.
Answer:
D. y = 5 + x
This problem shows the amount of library books Matt will have after any random amount of weeks he checked out one book, combined with the 5 he had before starting this pattern.
