For this case, the first thing we must do is define a variable.
We have then:
x: number of weeks
y: amount of cereal that Melanie eats
We now write the function that models the problem:

Then, for 5 weeks we have:
Answer:
A model that best represents the number of pounds of cereal she eats is:
For 5 weeks we have:
Account balance is not proportional to the number of weeks.
You can use the prefixes to figure this out:
Mono: one
Bi: two
Tri: three
So, because there is only one term, it is a monomial.