# Lesson 2

### Lesson Purpose

The purpose of this lesson is for students to find the value of unknown addends by adding or subtracting within 100.

### Lesson Narrative

In a previous lesson, students added and subtracted within 100 without composing or decomposing a ten to solve Compare problems. In this lesson, students add or subtract within 100 without composing or decomposing a ten and share the methods they use to find unknown addends. During each synthesis, students share methods based on place value and discuss the relationship between addition and subtraction.

This lesson also serves the purpose of introducing base-ten blocks as a math tool that can be used to represent larger numbers and computation methods based on place value. Throughout the lesson, students have opportunities to describe the usefulness of this new tool in comparison to connecting cubes and other representations they may use to find unknown values (MP5).

• Engagement
• MLR8

Activity 1: How Did You Find It?

### Learning Goals

Teacher Facing

• Describe their methods using place value understanding.
• Find the unknown addend in equations within 100.

### Student Facing

• Let’s find values that make the equations true.

### Required Materials

Materials to Gather

### Required Preparation

Activity 1:

• Each group of 2 needs 90–100 connecting cubes, but no more than 35 towers of 10 cubes should be included in their collection. Break apart any extra towers for this activity.

Building Towards

### 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

Who got to do math today in class and how do you know? Identify the norms or routines that allowed those students to engage in mathematics. How can you adjust these norms and routines so all students do math tomorrow?

### Suggested Centers

• Capture Squares (1–3), Stage 1: Add within 10 (Supporting)
• Five in a Row: Addition and Subtraction (1–2), Stage 5: Add within 100 without Composing (Supporting)