Lesson objective: Understand that we can add two-digit numbers using our understanding of place value.

Students bring prior knowledge of counting by tens for computation from 1.OA.6. This prior knowledge is extended to counting on by both tens and ones as students use place value models to solve additon problems. A conceptual challenge students may encounter is understanding when to compose a ten and what this means in finding a sum.

The concept is developed through work with rods and units, which helps students experience the concrete level of addition in situations when composing a ten may be necessary.

This work helps students deepen their understanding of number, because students represent relationships among numbers by composing and decomposing 10s and 1s to add and subtract.

Students engage in Mathematical Practice 7 as they look for and make use of structure, which is central to the work in this task. Students should identify the structure of place value in order to add or subtract within 100. They will use place value tools, such as rods and units, to model addition situations. The use of these tools will help students understand unitizing (ten ones regrouped into one ten).

**Key vocabulary:**

- compose
- counting on
- tens
- ones
- place value

**Special materials needed:**

- place value tools (rods and units)
- unifix or snap cubes