The cell phone plan from Company C costs $15 per month, plus $12 per gigabyte for data used. The plan from Company D costs $75 per month, with unlimited data. Rule C gives the monthly cost, in dollars, of using gigabytes of data on Company C’s plan. Rule gives the monthly cost, in dollars, of using gigabytes of data on Company D’s plan.
Sara wants to know what company to go with if her average gigabyte usage is 7 gigabytes per month. Explain which plan she should use, Company C or D? Make sure you support your choice with mathematical evidence.