This lesson is optional because it presents an open-ended problem for students to review everything they have learned in this unit. Students invent their own situation that involves a proportional relationship. They represent it using a sentence, a table, an equation, and a graph. As students identify quantities and relationships to represent a situation, they are modeling with mathematics (MP4).

Note that the real-world relationship that students choose to represent may not be exactly proportional. In other words, the rate relating the two quantities may involve some variability and is only roughly constant. It is fine for students to approximate an average constant rate in such cases. Discussions of measurement error and statistical variability will be addressed in later units.