# Lesson 9

The Difference Between Numbers

### Lesson Purpose

The purpose of this lesson is for students to represent and compare different subtraction methods on the number line.

### Lesson Narrative

In previous lessons, students represented addition and subtraction equations on the number line and wrote equations to match representations on a number line.

In this lesson, students deepen their understanding of subtraction as taking from and as an unknown addend problem. Students represent subtraction methods on the number line and compare how each representation shows the difference. The number line is also used to help students visualize when counting on and counting back strategies may be useful when subtracting 2 two-digit numbers. For example, $$17 - 14$$ can be solved by starting at 17 and counting back 14. It can also be solved by starting at 14 and counting on 3. In this case, counting on would likely be helpful because the numbers are close together and the structure of the number line helps show why this strategy is effective (MP7).

• Action and Expression

### Learning Goals

Teacher Facing

• On a number line, represent counting on and counting back strategies for solving subtraction equations.

### Student Facing

• Let’s represent ways to subtract on the number line.

### Required Materials

Materials to Gather

Materials to Copy

• Number Line to 100

### Required Preparation

Activity 1:

• Place the number line recording sheets in sheet protectors. They will be used in the next activity and future lessons.

### 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

Which students had opportunities to share their diagrams and thinking during whole-class discussion? How did you select these students?

### Suggested Centers

• Number Puzzles: Addition and Subtraction (1–4), Stage 4: Within 100 with Composing (Addressing)
• Number Line Scoot (2–3), Stage 1: Twos, Fives, and Tens (Addressing)