# Lesson 21

Compose and Decompose Numbers 11–19

### Lesson Purpose

The purpose of this lesson is for students to compose and decompose numbers 11–19 using 10 ones and some more ones.

### Lesson Narrative

In the first activity, students decompose groups of 11–19 objects into groups of 10 ones and some more ones. Then students determine the missing part to compose numbers 11–19.

If students need additional support with the concepts in this lesson, refer back to Unit 6, Section B in the curriculum materials.

- Action and Expression

- MLR8

Activity 1: Where Will They Sit?

### Learning Goals

Teacher Facing

- Compose and decompose numbers 11–19 using 10 ones and some more ones.

### Student Facing

- Let’s make groups of 10.

### Required Materials

### Required Preparation

Activity 1:

- Each group of 2 needs access to at least 2 collections of 11–19 objects.

Activity 2:

- Students can use their bead tool from a previous lesson.

Activity 3:

- Gather materials from:
- Shake and Spill
- Number Race
- Grab and Count
- What's Behind My Back?
- Pattern Blocks

### Lesson Timeline

Warm-up | 10 min |

Activity 1 | 15 min |

Activity 2 | 10 min |

Activity 3 | 20 min |

Lesson Synthesis | 5 min |

Cool-down | 0 min |

### Teacher Reflection Questions

As you finish up this unit, reflect on the norms and activities that have supported each student in learning math. How have you seen each student grow as a young mathematician throughout this work? How have you seen yourself grow as a teacher?