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. Set up Desta’s monthly average cost function and find her average cost per month be after being on the plan for 1 year.