4. A cell phone company offers two monthly plans. Subscribers to Plan A pay a monthly
fee of $15, and are charged 4 cents for every minute of cell phone usage. Subscribers
to the second plan, Plan B, pay a monthly fee of $10, and are charged 5 cents for
every minute of cell phone usage. Martha can afford no more than $40 per month for
a cell phone plan. Which plan should she choose?