Areal 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 $.44 a minute for daytime calls and $.19 a minute for evening calls. plan b has a charge of $.54 a minute for daytime calls and $.15 a minute for evening calls. plan c has a flat rate of $80 with 225 minutes of calls allowed per month and a charge of $.40 per minute beyond that, day or evening.