# Lesson 14

Think Before You Subtract

### Lesson Purpose

The purpose of this lesson is for students to analyze expressions to determine if a unit will be decomposed before subtracting.

### Lesson Narrative

In a previous lesson, students subtracted from a three-digit number and decomposed a ten or a hundred to subtract by place. Students represented their thinking with base-ten blocks, drawings, words, or numbers and explained the steps of their method.

In this lesson, students subtract two-digit numbers from three-digit numbers when 2 units are decomposed. Students are encouraged to attend to the details of the numbers in each expression to decide whether or not any units will need to be decomposed before subtracting (MP7). Throughout the lesson, students explain their reasoning and critique their peers' reasoning as they use their understanding of place value to analyze expressions and plan their methods (MP3).

• Action and Expression

Activity 1: Agree to Disagree

### Learning Goals

Teacher Facing

• Subtract a two-digit number from a three-digit number using place value strategies that include decomposing 2 units.

### Student Facing

• Let’s think about decomposing before we subtract.

### Required Materials

Materials to Gather

### 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

What connections did students make between the different methods and reasoning shared in today's lesson? What evidence are you seeing that students' are using their understanding of place value to make sense of expressions and other's methods for subtraction?