Understand subtraction as an unknown-addend problem. For example, subtract 10 – 8 by finding the number that makes 10 when added to 8. Add and subtract within 20.



Students need to recognize the importance of knowing that addition and subtraction are related to one another. Solving for unknown portions in a number sentence is a foundational skill for later algebra studies.  This standard refers specifically to the useful strategy to assist students in solving subtraction problems that involves using a related addition fact. Practice with this strategy will allow for more fluency between addition and subtraction.

Understanding the Standard:

    • Fact families are especially beneficial in helping students to understand the relationship between addition and subtraction.
    • Students should have ample opportunities to use manipulatives and technologies in constructing concrete models of subtraction problems.
    • Visuals, such as drawings, models, and pictures, can be used to further develop understanding the processes involved in using an addition sentence to compute the difference in a subtraction problem.

      Questions to Focus Instruction:

        • Are students able to use an addition sentence when solving for the difference in a subtraction problem?
        • Can students model a subtraction problem using concrete manipulatives and visuals to explain how to use a related addition fact to solve for the difference?


          Prior to: Students are able to count fluently through 20. Students can read and recognize numbers and the word forms of those numbers through 20 using the think addition strategy. Students can add and subtract fluently through 10. Go to K.OA.A.4  to see previous skills in this progression.

          At Grade Level:
          Students can add and subtract fluently through 20. Students can construct “fact families” that show the relationship that exists between addition and subtraction. When presented with a subtraction problem, students are familiar with computing the missing addend as a strategy for the solution.

          Moving Beyond: Students will use this skill to solve for more complex addition and subtraction problems through 100 and involving two-digit numbers. Go to 3.OA.B.6 to see the progression of related skills.