Unit 7, Lesson 14
Think Before You Subtract
Grade 2
Preparation
Lesson Narrative
In a previous lesson, students decomposed a ten or a hundred to subtract three-digit numbers 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 two 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
None
- Explain (orally) strategies for subtracting a two-digit number from a three-digit number, based on place value.
- Explain (orally) when one or more units will be decomposed when subtracting by place.
- Let’s think about decomposing before subtracting.
Required Preparation
None
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 others’ methods for subtraction?