Lesson 14
Think Before You Subtract
Lesson Purpose
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
Required Preparation
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
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)