Problem $9$

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 $$\mathrm{.}45$ a minute for

daytime calls and $$\mathrm{.}20$ a minute for evening calls. Plan B has a

charge of $$\mathrm{.}55$ a minute for daytime

calls and $$\mathrm{.}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 $$\mathrm{.}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.