A real estate agent is considering changing her cell phone plan. There are three plans to choose from, all of which involve a monthly service charge of $20. Plan A has a cost of $.45 a minute for daytime calls and $.20 a minute for evening calls. Plan B has a charge of $.55 a minute for daytime calls and $.15 a minute for evening calls. Plan C has a flat rate of $80 with 200 minutes of calls allowed per month and a charge of $.40 per minute beyond that, day or evening.

a. Determine the total charge under each plan for this case: 120 minutes of day calls and 40 minutes of evening calls in a month.

b. Prepare a graph that shows total monthly cost for each plan versus daytime call minutes only.

c. If the agent will use the service only for daytime calls, over what range of call minutes will each plan be optimal?