# Lesson 3

Relacionemos la suma y la resta hasta 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

• Encontremos el número que hace que ecuaciones con 20 sean verdaderas.

### Required Materials

Materials to Gather

Materials to Copy

• What's Behind My Back Stage 3 Recording Sheet, Spanish

### 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)