Desta wants to purchase “The Ultimate Plan” from her cell phone provider.

Desta wants to purchase “The Ultimate Plan” from her cell phone provider. If she enrolls in this plan, she can purchase the latest cell phone from the provider for $250. The plan consists of unlimited talk and text and 18GB of data for $100 per month. It will cost an extra $20 per month to add her husband’s phone to the plan and an extra $10 per month to have her tablet connected.
a. Set up Desta’s monthly cost function for this plan. Let x represent the number of months she has been on the plan.
b. Set up Desta’s monthly average cost function.
c. What will her average cost per month be after being on the plan for 3 months?
d. What will her average cost per month be after being on the plan for 1 year?
e. Approximately how many months does Desta have to be in the plan for her average monthly cost to be $140?