Lesson 3

In this lesson, students continue to work with proportional relationships represented by tables using contexts familiar from previous grades: unit conversion and constant speed. They recognize the constant of proportionality as the conversion factor or the speed, and use it to answer questions about the context. Although students might continue to reason with equivalent ratios to solve problems, the contexts are designed so that it is more efficient to use the constant of proportionality. For example, when converting length measurements from feet to inches, it is more convenient to know the rule “multiply by 12” than to use an equivalent ratio with a different scale factor every time: “1 foot is 12 inches, so multiplying both quantities by 3 I see that 3 feet is 36 inches, and multiplying both quantities by 5 I see that 5 feet is 60 inches.”