A real estate agent is considering changing her cell phone plan. There are 3 plans to choose from all which have a monthly service fee of $20. Plan A has a rate of $.45 cents for daytime min and $.20 for evening. Plan B $.55 day and $.15 evening. Plan C has a flat rate of $80 for 200 minutes of calls and a charge of $.40 beyond that 200 if either day or night. what is algebraic proof and optimal plan?