Lesson objective: Fluently add and subtract within 20 using mental strategies.

Students bring prior knowledge of using place value understanding and properties of operations to add and subtract from 2.NBT.B. This prior knowledge is extended to include mental calculations as students use and explain strategies for accurately and efficiently adding and subtracting within 20. A conceptual challenge students may encounter is understanding that different strategies are more efficient than others depending on the numbers used in the equation. Students may feel comfortable with one strategy and try to apply it to all equations.

The concept is developed through work with justifying and explaining strategy choice, which allows students to demonstrate their understanding and flexibility with using all previously learned strategies.

This work helps students deepen their understanding of operations because they are extending their understandings of properties of operations through repeated experiences with addition and subtraction.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they draw from their own toolkit for solving problems and utilize strategies that have proven to be efficient for them. In this lesson, it will take the form of children determining what strategy would work best for the given situations. Mathematical Practice 6 (Attend to precision) is also a critical practice within this task as students apply strategies that they have been practicing all year. They are focusing on the precision of communication, in speech, as well as written form. They will be explaining, justifying, and supporting their choice of strategies as they discuss their reasoning with peers.

**Key vocabulary:**

- efficiently
- fluently
- mental calculations