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Common Core Math1st GradeOperations & Algebraic Thinking

1.OA.8: Find the Unknown Number in an Equation

Also written as 1.OA.D.8 and CCSS.MATH.CONTENT.1.OA.D.8 · Cluster: Work with addition and subtraction equations.

Make a Missing Number WorksheetAll 21 1st Grade Math Standards

The Official Wording

Determine the unknown whole number in an addition or subtraction equation relating three whole numbers. For example, determine the unknown number that makes the equation true in each of the equations 8 + ? = 11, 5 = _ - 3, 6 + 6 = _.

Common Core State Standards, Grade 1, Operations & Algebraic Thinking · Read it at the source · Checked October 14, 2026

What 1.OA.8 Means in Plain English

First graders find the number hiding behind a box in an equation with three numbers, wherever the box sits: 6 + ? = 13, ? - 4 = 9, or 15 = 7 + ?. A related fact or counting up finds it.

The hardest cases put the box first, as in ? - 4 = 9, or put the total on the left. Students who solve those are ready for 2nd grade word problems within 100 with unknowns anywhere (2.OA.1), and for 3rd grade unknown factors (3.OA.4).

1.OA.8 I Can Statements

  • I can find the missing number in an addition equation.
  • I can find the missing number in a subtraction equation.
  • I can check my answer by putting it back in.

1.OA.8 Worked Example

The trickiest box position in 1st grade, the start of a subtraction, solved by putting the pieces back together.

Problem

What number makes ? - 4 = 9 true?

The box comes first
  1. Read it as a story: some cubes, 4 taken away, 9 left. The start is missing.? - 4 = 9
  2. The 9 left and the 4 taken away were both part of the start. Put them back together.9 + 4 = ?
  3. Make a ten: 9 and 1 is 10, then 3 more.9 + 1 + 3 = 13
  4. Write the number in the box and read the equation.13 - 4 = 9
AnswerThe unknown number is 13

Check it: Start with 13 cubes and take 4 away. 9 are left, so 13 - 4 = 9 is true.

Common Mistakes with 1.OA.8

Using the two numbers you can see

? - 4 = 9 answered with 5, because 9 - 4 = 5

Try this: Have the student test the answer in the box: does 5 - 4 make 9? A false equation means try again.

Adding everything in sight

7 + ? = 12 answered with 19

Try this: Read the equation aloud as a question, 7 and how many more make 12, and count up on fingers from 7.

Stuck when the total comes first

Leaves 12 = 5 + ? blank: “it’s the wrong way round”

Try this: Rewrite it the familiar way round, 5 + ? = 12, then return to the original to show the same box holds 7.

Where 1.OA.8 Fits

Comes BeforeK.OA.4Find the number that makes 10 with any number from 1 to 9.
Alongside1.OA.7Know what = means and tell true equations from false ones.1.OA.4Solve a subtraction by finding the missing addend.1.OA.1Solve word problems within 20 with the unknown in any position.
Comes Next2.OA.1Solve word problems within 100 with a symbol for the unknown.3.OA.4Find the unknown number in a multiplication or division equation.

Practice 1.OA.8

Math Worksheet GeneratorFree ToolMissing-number equations within 20 at the 1st grade level, with the box at the start, in the middle, at the end and on the left of =.Grade 1Missing numberAnswer key
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Quick Check

  1. What number makes 7 + ? = 15 true?
  2. What number makes ? - 6 = 8 true?
  3. What number makes 10 = 13 - ? true?

1.OA.8: Other Names

1.OA.81.OA.D.8CCSS.MATH.CONTENT.1.OA.D.8

Common Core State Standards © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved. Official wording from the Common Core State Standards PDFs at corestandards.org, last checked October 14, 2026. The explanation, I can statements, example, mistakes and practice on this page are EMStudio’s own, written beside the official wording. They are not part of the standard.