# Lesson 12

Decompose to Subtract

### Lesson Purpose

The purpose of this lesson is for students to decompose a ten in order to subtract within 1,000.

### Lesson Narrative

In previous units, students used methods based on place value to decompose a ten when subtracting within 100. In this lesson, students extend their understanding of decomposing tens to subtract by place within 1,000. In the first activity, students are invited to use whatever method makes sense to them to subtract one- and two-digit numbers from a three-digit number. In the following activity, they use base-ten blocks to subtract from a three-digit number and reason about when a ten is decomposed when subtracting by place.

• Engagement

### Learning Goals

Teacher Facing

• Subtract numbers within 1,000 using place value strategies that include decomposing a ten.

### Student Facing

• Let’s subtract within 1,000.

### 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

In previous units, students decomposed a ten when subtracting two-digit numbers. How did you see students use this understanding as they subtracted from three-digit numbers in this lesson?

### Suggested Centers

• How Close? (1–5), Stage 4: Add to 1,000 (Addressing)
• Five in a Row: Addition and Subtraction (1–2), Stage 8: Add within 1,000 with Composing (Addressing)