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

Students bring prior knowledge of composing and decomposing 10 from K.OA.A.4 This prior knowledge is extended to understanding that addition and subtraction are related as students work on proving that addition and subtraction are inverse operations. A conceptual challenge students may encounter is knowing when to use addtion to solve for an unknown number to solve a subtraction problem.

The concept is developed through work with a part-part-whole diagram.

This work helps students deepen their understanding of operations because there is a relationship between addition and subtraction.

Students engage in Mathematical Practice 7 (Look for and make use of structure). Students will see similarities in operational problem types such as compare problems, join together problems, and separate problems. They will use a part-part-whole diagram to show that subtraction can be thought of as an unknown-addend problem because of the inverse relationship between addition and subtraction.

**Key vocabulary:**

- addend
- diagram
- equation
- number partners
- inverse operation

**Special materials needed:**

- counters or snap cubes
- part-part-whole diagrams