Lesson objective: Add or subtract to find the missing addend.

This lesson helps to build procedural skill with solving for the missing addend. A variety of strategies and tools are used here because they support student understanding. This work develops students' understanding that addition and subtraction are related.

Students engage in Mathematical Practice 2 (reason abstractly and quantitatively) as they solve for the missing addend.

**Key vocabulary:**

- addend
- decompose
- make a ten
- strategy
- tool

**Special materials needed:**

- student number lines (optional)