The graph shows the cost, in dollars, of a phone call for different call durations. The x-axis represents call length (minutes), and the y-axis represents call cost.

What is the average rate of change?

Please directly answer the question and provide the final value, e.g., 1, 2.5, 300.
Question: The graph shows the cost, in dollars, of a phone call for different call durations. The x-axis represents call length (minutes), and the y-axis represents call cost.

What is the average rate of change?

Please first conduct reasoning, and then answer the question and provide the final value, e.g., 1, 2.5, 300, at the end.
Question: The graph shows the cost, in dollars, of a phone call for different call durations. The x-axis represents call length (minutes), and the y-axis represents call cost.

What is the average rate of change?
