Lesson 9

Add and Subtract Within 100

Lesson Purpose

The purpose of this lesson is for students to add and subtract within 100, including composing and decomposing a ten, using strategies based on place value and the properties of operations.

Lesson Narrative

In previous lessons, students explored different methods for addition and subtraction with and without composing or decomposing a ten. Students used base-ten blocks, drawings, and equations to represent their methods.

In this lesson, students choose their preferred methods and representations to add and subtract. Throughout the lesson, students are asked to connect expressions and diagrams, choose their own methods for adding and subtracting, and make sense of others thinking (MP2, MP3, MP6). Listen for the ways students explain their methods to others and look for ways to help students provide feedback to one another when their representations or explanations are not clear.

• Representation
• MLR8

Learning Goals

Teacher Facing

• Add and subtract within 100 using strategies based on place value, including composing and decomposing a ten, and the properties of operations.

Student Facing

• Let’s find the difference in our own way.

Required Materials

Materials to Gather

Materials to Copy

• Sort and Find the Value

Required Preparation

Activity 1:

• Create a set of cards from the blackline master for each group of 23.

Building Towards

Lesson Timeline

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

Teacher Reflection Questions

Reflect on whose thinking was heard today. Reflect on whose thinking was not heard but could have enriched the conversations. What prompts or structures might better enable the latter to share their voices and reasoning?

Suggested Centers

• Capture Squares (1–3), Stage 3: Add within 20 (Addressing)
• Five in a Row: Addition and Subtraction (1–2), Stage 6: Add within 100 with Composing (Addressing)
• Target Numbers (1–5), Stage 4: Subtract Tens or Ones (Addressing)