Lesson objective: Students will understand that number lines can be used to model addition and subtraction problems related to length.

Students bring prior knowledge of the structure of a number line diagram from 2.MD.B.6. This prior knowledge is extended to addition and subtraction as students show multiple lengths on the same number line, and use a number line a tool when adding or subtracting lengths. A conceptual challenge students may encounter is understanding when it is appropriate within the context of a problem to draw the lengths starting at zero, and when it is appropriate to draw the lengths end-to-end.

The concept is developed through work with a number line diagram, which serves as a measurement model with which students can use strategies relating to distance, proximity of numbers, and reference points to reason about addition and subtraction.

This work helps students deepen their understanding of operations because the number line diagram prompts students to count forward and backward in order as they add and subtract.

Students engage in Mathematical Practice 5 (Use appropriate tools strategically) as they strategically use number lines as a tool to anchor addition and subtraction work.

**Key vocabulary:**

- number line diagram
- addend

**Special materials needed:**

- blank number line diagrams (optional)