# Lesson 21 Compose and Decompose Numbers 11–19

Let’s make groups of 10.

## Warm-up Which One Doesn’t Belong: Numbers 11–20

Which one doesn’t belong?

## Activity 1 Where Will They Sit?

=

=

=

=

## Activity 2 Finish the Equations

Kiran wrote equations to show the total number of students and how many students sat at the table and how many sat on the rug, but he didn’t finish the equations. Finish filling in each equation. You can use connecting cubes or two-color counters if they are helpful.

## Activity 3 Centers: Choice Time

Choose a center.

Shake and Spill

Number Race

Grab and Count

What’s Behind My Back?

Pattern Blocks