Mystery Addends: Representing Word Problems

  • First Grade, Second Grade
  • Math
  • 60 minutes
  • Standards: 2.OA.A.1, 1.NBT.C.4
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September 5, 2015
by Dwayne Slobodnick

Make a math mystery! In this lesson, help your students understand the relationship between addition and subtraction and how a missing addend word problem is represented with a number sentence.

Learning Objectives

Students will be able to use mental math to solve missing addend questions using both addition and subtraction strategies. Students will be able to represent a given missing addend word problem using a number sentence. Students will be able to identify the initial, change, and result of a word problem.

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Introduction (10 minutes)

  • Tell the students that today they will be reviewing fact families, or related numbers, and they will use that knowledge to help them represent missing addend word problems.
  • Remind your students that an addend is a number in a problem that is being added.
  • Show the example of a fact family triangle to the students, and have them explain the relationship between the 3 numbers.
  • Tell the students that they will make their own fact family triangles and play a game with them.
  • Pass out the triangles, and have the students each write out a fact family.
  • Then, have them walk around the room to find a partner to answer the question.
  • Direct each student to cover up one of the 3 numbers and show it to his partner.
  • Next, have your students find new partners and do it again.
  • After they have asked several peers, have them sit down.
  • Instruct your students to keep the triangles in their desks to play later.

Explicit Instruction/Teacher Modeling (10 minutes)

  • Write a word problem on the board similar to one on the worksheet.
  • Read it aloud to the class and explain your thinking to them, reasoning how the sentences tell you the starting and ending amounts.
  • Use the key terms initial, or beginning, and change, or difference, when discussing the missing addends to help the students visualize the story.

Guided Practice/Interactive Modeling (10 minutes)

  • Write a new word problem on the board similar to the one during the explicit instruction.
  • This time, have the class read out the question, and have students go through sentence by sentence to locate the addends for the initial and change.
  • Ask the students to help you write down the number sentence that represents the word problem using a ∆ (triangle) to represent the mystery addend.
  • Then, change the word problem a little to switch the mystery addend, and then have the students explain how they would answer that one.

Independent Working Time (20 minutes)

  • Explain to the students that they will independently do what they just practiced with you on the board.
  • Handout the Missing Addend Word Problem worksheets, and have the students cut out the number sentences and match them to the word problems.
  • When students finish this, instruct them to pair up and discuss why they chose the number sentence they did for each word problem.
  • As the students are doing this, walk around and conference with students and document their thinking.
  • When you notice that the students have done their pair-sharing, choose some students to share what they did and why.



  • Enrichment: For advanced students, ask them to make their own missing addend word problems, and have their peers write number sentences to represent them.
  • Support: Give students manipulatives, or instruct them to draw a picture of the word problem to help them visualize it.

Related Books and/or Media


Assessment (5 minutes)

  • Make sure your students are correctly representing the missing addend word problems with number sentences.
  • Walk around as your students are completing the assignment, checking their initial and change amounts.

Review and Closing (5 minutes)

  • After the students share their answers as a class, write down a couple of number sentences on a piece of chart paper to show missing addends.

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