Lesson 2
Find the Unknown Addend
Lesson Purpose
Lesson Narrative
In a previous lesson, students added and subtracted within 100 without composing or decomposing a ten to solve Compare problems. In this lesson, students add or subtract within 100 without composing or decomposing a ten and share the methods they use to find unknown addends. During each synthesis, students share methods based on place value and discuss the relationship between addition and subtraction.
This lesson also serves the purpose of introducing base-ten blocks as a math tool that can be used to represent larger numbers and computation methods based on place value. Throughout the lesson, students have opportunities to describe the usefulness of this new tool in comparison to connecting cubes and other representations they may use to find unknown values (MP5).
- Engagement
- MLR8
Activity 1: How Did You Find It?
Learning Goals
Teacher Facing
- Describe their methods using place value understanding.
- Find the unknown addend in equations within 100.
Student Facing
- Let’s find values that make the equations true.
Required Materials
Materials to Gather
Required Preparation
Activity 1:
- Each group of 2 needs 90–100 connecting cubes, but no more than 3–5 towers of 10 cubes should be included in their collection. Break apart any extra towers for this activity.
Lesson Timeline
| Warm-up | 10 min |
| Activity 1 | 20 min |
| Activity 2 | 15 min |
| Lesson Synthesis | 10 min |
| Cool-down | 5 min |
Teacher Reflection Questions
Suggested Centers
- Capture Squares (1–3), Stage 1: Add within 10 (Supporting)
- Five in a Row: Addition and Subtraction (1–2), Stage 5: Add within 100 without Composing (Supporting)