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

This lesson helps to build fluency with adding/subtracting within 20 using mental strategies. The game "What's My Number?" is used here because it requires students to both add and subtract to find an unknown number. This work develops students' understanding that knowing some addition and subtraction facts from memory helps to solve new addition and subtraction problems.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they utilize strategies they have been practicing all year to determine an unknown in a problem. Students are analyzing the sum and one known addend to determine the unknown addend.

**Key vocabulary:**

- addend
- difference
- mental calculations
- strategy
- sum

**Special materials needed:**

- Playing cards with Kings, Queens, Jacks, Jokers, and Aces removed