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 $.38 a minute for daytime calls and $.18 a minute for evening calls. Plan B has a charge of $.48 a minute for daytime calls and $.14 a minute for evening calls. Plan C has a flat rate of $75 with 250 minutes of calls allowed per month and a charge of $.36 per minute beyond that, day or evening.
If the agent will use the service for daytime calls, over what range of call minutes will each plan be optimal?
Suppose that the agent expects both daytime and evening calls. At what point (i.e., percentage of total call minutes used for daytime calls) would she be indifferent between plans A and B?