# Lesson 1

Sums and Differences Within 20

### Lesson Purpose

The purpose of this lesson is for students to fluently add and subtract within 20 using mental strategies.

### Lesson Narrative

In this lesson, students find the value of sums and unknown addends from memory in expressions with values within 20. In the first activity, students complete an inventory of sums within 20 and identify those sums that they do not know yet from memory. In the second activity, they continue to practice fluency with addition and subtraction within 20. Throughout the lesson, encourage students to share the methods they use to recall or find a sum or difference fluently, including methods based on using the relationship between addition and subtraction, decomposing to get to a ten, and creating expressions that are equivalent, but easier to find mentally.

• Action and Expression

### Learning Goals

Teacher Facing

• Fluently add and subtract within 20.

### Student Facing

• Let’s find the value of sums and differences mentally.

### Required Materials

Materials to Gather

Materials to Copy

• Spin and Find the Missing Number Spinners

### Lesson Timeline

 Warm-up 10 min Activity 1 20 min Activity 2 15 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

Which sums within 20 did students show they are still working on? How can you make time for practice with these sums during the school day?

### Print Formatted Materials

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