Lesson objective: Find the difference between lengths using a number line.

This lesson helps to build procedural skill with whole number subtraction. A number line diagram is used here because it highlights part-whole relationships and supports the strategy of "counting on" to find a missing addend or "counting back" to subtract. This work develops students' understanding that we can use a number line to solve many types of addition and subtraction problems related to length.

Students engage in Mathematical Practice 6 (Attend to precision) as they label number line diagrams precisely. They also engage in Mathematical Practice 7 (Look for and make use of structure) as they look for number patterns and relationships on the number lines to develop computational strategies.

**Key vocabulary:**

- addend
- count back
- count on
- difference
- number line diagram

**Special materials needed:**

- blank number lines (optional)