Lesson objective: Understand that subtraction can be thought of as an unknown addend problem.

This lesson helps to build strategies with addition and subtraction. Part-part-whole diagrams and number lines are used here because they show the number partners in related addition and subtraction facts. This work develops students' understanding that subtraction can be thought of as an unknown addend problem.

Students engage in Mathematical Practice 7 (Look for and make use of structure) as they prove the inverse relationship between addition and subtraction.

**Key vocabulary:**

- inverse operation
- unknown addend

**Special materials needed:**

- number line
- counters