Lesson plan

Composing and Decomposing

Use this lesson to introduce composing and decomposing as part of your students’ addition and subtraction toolkits.
Need extra help for EL students? Try the Decomposing and Adding Place Values pre-lesson.
EL Adjustments
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Need extra help for EL students? Try the Decomposing and Adding Place Values pre-lesson.

Students will be able to compose and decompose numbers within 20.

The adjustment to the whole group lesson is a modification to differentiate for children who are English learners.
EL adjustments
(2 minutes)
  • Ask students if they know the meaning of compose. Write it on the chart paper. Define it as coming together to make something. For example, Four is composed of four ones.
  • Ask students if they can now guess the meaning of decompose. Write it on the chart paper. Define it as to separate into smaller or simpler parts. For example, the number 42 can be decomposed into four tens and two ones.
  • Explain that today students will be composing and decomposing numbers within 20. Elaborate that these skills will help us add and subtract.
(8 minutes)
  • Distribute student whiteboards and markers.
  • Instruct students to follow along on their whiteboards with you as you draw on the chart paper. Draw a number bond: a circle on the top, with two circles underneath connected to the top circle with lines. In the bottom circles, write the numbers 6 and 3, respectively.
  • Ask students to turn and tell their partner the sum of 6 and 3. Say, “What is composed of 6 and 3?”
  • Record the number 9 in the top circle.
  • Instruct students to erase the numbers in their number bond while you draw a new one. Write the number 14 in the top circle, and leave the bottom two circles blank.
  • Ask students to turn and tell their partner the easiest way to decompose the number 14. Explain that there is more than one correct way.
  • Ask student volunteers to share their strategies for decomposition. Choose one and record it on your paper.
(10 minutes)
  • Distribute the Compose and Decompose Within 20 worksheet.
  • Read the instructions and example problem aloud.
  • Have students work in partnerships to complete the worksheet.
(15 minutes)
  • Distribute the Decompose to Add: Two Digits Within 20 worksheet.
  • Explain that, instead of number bonds, they will be breaking down the numbers in each problem into smaller chunks. Review the example.
  • Instruct students to begin working independently.
  • Circulate the room while students are working, answering questions and informally assessing student abilities.


  • Challenge students to complete the Subtraction with Decomposition worksheet.


  • Review simple decomposition with the Break the Numbers Down worksheet.
  • Call students who need extra support to work in a small group with you.
(5 minutes)
  • Assess student abilities informally through class discussion and independent work time.
  • Have students turn in their worksheets for you to review for understanding and accuracy.
(5 minutes)
  • Have students think-pair-share with a neighbor, answering one of the following sentence stems:
    • A new word I learned today was ____.
    • My favorite part of the lesson was ____.
  • Ask for 2–3 student volunteers to share their partner’s response.

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