Lesson objective: Solve problems by using a number line to add and subtract lengths.

This lesson provides an opportunity for students to apply their knowledge and understanding of addition and subtraction to a real-life situation. Students are asked to plan the arrangement of shelves on a bookcase given the heights of the books that need to fit on the shelves, and then create a diagram to show their arrangements.

Key Concept students will use:

- a number line to solve many types of addition and subtraction related to length.

Skills students will use:

- representing numbers as lengths on a number line.
- using a number line diagram as a tool to add and subtract lengths.

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

**Key vocabulary: **

- number line diagram
- count on/count back

**Special materials needed:**

- blank number lines (optional)