In this lesson, students build fluency with subtracting signed numbers. They learn that “the difference of and ” means . They see that the difference between two numbers can be positive or negative depending on the relative position of the numbers on the number line. For example, , while . If students had previously understood subtraction to involve always subtracting the lesser number from the greater number, then this lesson is an opportunity for them to develop a more nuanced understanding of subtraction. As students write subtraction expressions to represent differences in elevation, they are reasoning abstractly and quantitatively (MP2).

While this lesson emphasizes finding differences between signed numbers, the last activity “Phone Inventory” also gives students an opportunity to apply what they have learned about adding and subtracting signed numbers to represent situations and solve problems.