Lesson 3

Relate Addition and Subtraction within 20

Lesson Purpose

The purpose of this lesson is for students to find the number that makes equations true within 20.

Lesson Narrative

In this lesson, students apply the reasoning they used in previous lessons with addends that make 10 to find the number that makes an equations true that include the number 20. In the first activity, students learn stage 3 of the What's Behind My Back center in which they work with 20 cubes, organized into two towers of 10 cubes. In the second activity, students find the number that makes different equations with addition and subtraction expressions true. Throughout the lesson, students have opportunities to use what they know about the structure of whole numbers and the relationship of addition and subtraction to find the unknown numbers and explain their methods (MP3, MP7). 

Math Community

In the lesson synthesis, students use their Mathematical Community Poster to create norms for the classroom.

  • Representation
  • MLR7

Learning Goals

Teacher Facing

  • Find the number that makes equations within 20 true using the relationship between addition and subtraction.

Student Facing

  • Let’s find the number that makes equations with 20 true.

Required Materials

Materials to Gather

Materials to Copy

  • What's Behind My Back Stage 3 Recording Sheet

Required Preparation

CCSS Standards


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

What makes someone good at math? In what ways are you making assumptions about which of your students are good at math? 

Suggested Centers

  • Number Puzzles: Addition and Subtraction (1–4), Stage 1: Within 10 (Supporting)

Print Formatted Materials

Teachers with a valid work email address can click here to register or sign in for free access to Cool Down, Teacher Guide, and PowerPoint materials.

Student Task Statements pdf docx
Lesson Cover Page pdf docx
Cool Down Log In
Teacher Guide Log In
Teacher Presentation Materials pdf docx
Blackline Masters zip

Additional Resources

Google Slides Log In
PowerPoint Slides Log In