Lesson Plan

Mystery Addends: Representing Word Problems

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.
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Need extra help for EL students? Try the Circles in a Cup pre-lesson.
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Need extra help for EL students? Try the Circles in a Cup pre-lesson.

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.

The adjustment to the whole group lesson is a modification to differentiate for children who are English learners.
EL adjustments


(10 minutes)
Fact Families Triangle ExampleFact Families TrianglesMissing Addend Word Problems
  • 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.


  • Ask students to identify the larger number (18) and the smaller numbers (7 and 11). Model counting on from 11 to 18 as you think aloud, "11 + 7 = 18. If I know this fact, I also know that 7 + 11 = 18."
  • Use manipulatives or a number line to show students that 18 - 7 = 11 and 18 - 11 = 7.
  • Show students a few more examples of fact families before excusing them to make their own fact family triangles.


  • Remind students that fact families include two addition and two subtraction facts. The number at the top is the sum, or total, of the two parts in the bottom corners.
  • Write ________ + ________ = ________ and ________ - ________ = ________ on the board. Model solving for the third number in a fact family using both addition and subtraction.