This Instructional Activity is a part of the Lesson 3, Unit 6, Grade 8. Here, verbal information giving an initial value and a rate of change is translated into a function and its graph. Work through the example as a class. In part (b), explain why the value given in the question is the rate of change. It is a good idea to show this on the graph, demonstrating that each increase of unit for (miles) results in an increase of for (cost in dollars).