Like most teens, Kari enjoys texting with her friends. Her family's current plan gives her a certain base number of texts each month for a fixed monthly fee, and then charges per message beyond that number. That gets expensive if Kari texts way over the base number, so her parents are thinking of limiting her use or making her pay the extra message charge if she goes over. With the holidays approaching, Kari asked for the gift of a plan with unlimited texting. She doesn't know the details of their current plan, but she argued to her parents that with an unlimited plan they would be less stressed about how many texts she sends and they'd all save money. Her parents thought it over, and then gave her a challenge: if she can figure out the pricing of the current plan based on the last four monthly charges, they will upgrade to an unlimited plan. In August, she sent 212 texts and the bill was $19.91. During September, the number of texts dropped to 163 and the bill was $18.79. October was a busy month, with 304 texts and a charge of $23.13. Finally, in November she sent 264 texts and it cost $21.73.