# Lesson 12

### Lesson Purpose

The purpose of this lesson is for students to add 2 two-digit numbers, with composing a ten, using methods based on place value and properties of operations. Students write equations to represent addition methods.

### Lesson Narrative

In this lesson, students practice explaining their methods for adding two-digit numbers based on place value and the properties of operations (MP3). In the first activity, students are invited to choose any method that makes sense to them to add within 100 and explain their method to their peers. In the second activity, students use what they have learned about place value and methods for adding to create expressions that meet different constraints. Throughout the lesson, monitor for ways students are reasoning about place value and the structure of numbers (MP7).

Activity 3 is an optional activity that offers practice adding within 100 in real-world contexts.

• Representation
• MLR8

### Learning Goals

Teacher Facing

• Add 2 two-digit numbers using methods based on place value and properties of operations.
• Write equations to represent addition methods.

### Student Facing

• Let’s add two-digit numbers and write equations.

### Lesson Timeline

 Warm-up 10 min Activity 1 15 min Activity 2 20 min Activity 3 20 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

As you finish up this unit, reflect on the norms and activities that have supported each student in learning math. How have you seen each student grow as a young mathematician throughout this work? How have you seen yourself grow as a teacher? What will you continue to do and what will you improve on in Unit 6?