On average, Susan downloads songs per month. An online music vendor sells package prices for songs that can be downloaded onto personal digital devices. The graph below shows the package prices for the most popular promotions. Susan wants to know if she should buy her music from this company or pay a flat fee of per month offered by another company. Which is the better buy? a. Find the constant of proportionality for this situation.

b. Write an equation to represent the relationship.

c. Use your equation to find the answer to Susan’s question above. Justify your answer with mathematical evidence and a written explanation.