Lesson objective: Understand that we can use addition and subtraction facts we already know to solve new addition and subtraction facts.

Students bring prior knowledge of decomposing 10 from Grade K, Unit 12 K.OA.4. This prior knowledge is extended to decompose one addend in a problem and compose it with another addend to create a simpler equivalent equation. A conceptual challenge students may encounter is failing to apply the associative property to make the problem simpler.

The concept is developed through work with models like ten frames and snap cubes.

This work helps students deepen their understanding of operations because students apply the commutative and associative properties as they decompose and compose numbers to create simpler equivalent problems.

Students engage in Mathematical Practice #8 (students will look for and expresss regularity in repeated reasoning) as they use short cuts and numerical reasoning to effectively add and subtract. They build fluency and numeracy as they decompose and compose numbers to create easier problems.

**Key vocabulary:**

- decompose
- compose

**Special materials needed:**

- concrete manipulatives (i.e. snap cubes or ten frames)