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
Addressing
Lesson Timeline
Warmup  10 min 
Activity 1  20 min 
Activity 2  15 min 
Lesson Synthesis  10 min 
Cooldown  5 min 
Teacher Reflection Questions
Suggested Centers
 Number Puzzles: Addition and Subtraction (1–4), Stage 1: Within 10 (Supporting)
Print Formatted Materials
For access, consult one of our IM Certified Partners.
Additional Resources
Google Slides  For access, consult one of our IM Certified Partners. 

PowerPoint Slides  For access, consult one of our IM Certified Partners. 