1. **State the problem:** Jo has a mobile phone plan with a monthly charge of 16. She gets 150 free minutes and 150 free texts. After that, each additional minute costs 0.13 and each additional text costs 0.15. 2. **Define variables:** Let $m$ be the number of minutes used beyond 150. Let $t$ be the number of texts sent beyond 150. 3. **Write the cost function:** The total monthly cost $C$ is the base charge plus the cost for extra minutes and extra texts: $$ C = 16 + 0.13m + 0.15t $$ where $m \geq 0$ and $t \geq 0$. 4. **Explain the graph shape:** - For $m \leq 0$ and $t \leq 0$, cost is constant at 16. - For $m > 0$ or $t > 0$, cost increases linearly with slopes 0.13 and 0.15 respectively. 5. **Summary:** The cost function is piecewise linear with a flat region up to 150 minutes and texts, then increasing linearly. This models Jo's monthly phone charge accurately.