Missing Numbers & Unknowns | 1st Grade Math

Missing Numbers and Unknowns - Grade 1 Math
Grade 1 Math  ›  Addition & Subtraction  ›  Lesson 14
Grade 1 Math - Addition and Subtraction

Missing Numbers & Unknowns

Learn how to find missing numbers and unknowns in first grade addition and subtraction equations. Students practice missing addends, unknown totals, unknown differences, and unknown starting numbers within 20.

This lesson builds directly on understanding the equal sign: the missing number must make both sides of the equation have the same value. Use counting on, fact families, part-part-whole thinking, and the relationship between addition and subtraction to solve.

Missing numbers Unknown addends Addition within 20 Subtraction within 20 Grade 1 algebraic thinking

๐ŸŽฏ What You Will Learn

Find an unknown in different positions
The missing number can be a sum, an addend, a difference, the starting number, or the amount taken away.
Use addition to help with subtraction
Related addition facts can make subtraction unknowns easier to solve.
Use more than one strategy
Count on, use a fact family, think part-part-whole, or use a known fact.
Check the equation
Put your answer back into the equation and make sure both sides have the same value.
For parents & teachers — standards alignment: This lesson directly addresses CCSS 1.OA.D.8: determine the unknown whole number in an addition or subtraction equation relating three whole numbers. It also supports CCSS 1.OA.D.7 by using the equal sign as a relationship between equal values. Texas TEKS 1.5(E) reinforces this meaning of the equal sign, and TEKS 1.5(F) extends the unknown-number work to addition and subtraction equations with three or four terms. For that reason, a few clearly marked extension problems include expressions on both sides.

๐Ÿง  Key Vocabulary

unknown — the number we need to find.
missing number — another name for an unknown number in an equation.
addend — a number being added.
sum — the result of addition.
difference — the result of subtraction.
equation — a mathematical statement showing that two values are equal.

๐Ÿ”Ž Where Can the Unknown Be?

The unknown does not always come after the equal sign. It can appear in different places. The job is always the same: find the number that makes the equation true.

Unknown total 5 + 3 = ?
Unknown addend 5 + ? = 8
Unknown in subtraction 8 − ? = 3

๐Ÿงฉ Think in Parts and a Whole

Addition puts parts together to make a whole. Subtraction can help us find a missing part. If one part is 5 and the whole is 12, the missing part is 7.

Known part: 5
+
Missing part: ?
5 + ?
=
Whole: 12
Helpful connection: if 5 + ? = 12, then think about the related subtraction fact 12 − 5 = 7.

๐Ÿ› ️ Four Useful Strategies

Count on Start with a known number and count up to the total.
Use a fact family Switch between related addition and subtraction facts.
Think parts and whole Find the missing part or rebuild the whole.
Check Put the number back into the original equation.
Common mistake: do not automatically add the two visible numbers. First ask what the unknown represents. In 12 − □ = 5, the missing number is the amount taken away, not the difference.

๐ŸŒŸ 25 Worked Examples

Try each problem first. Then open “Show solution” to compare your strategy.
1 Find an unknown total
Find the missing number: 4 + 3 = □.
Show solution Hide solution

Add the two known parts: 4 + 3 = 7. So the missing number is 7. Check: 4 + 3 = 7.

2 Find a missing addend
Find the missing number: 3 + □ = 8.
Show solution Hide solution

Start at 3 and count on to 8: 4, 5, 6, 7, 8. That is 5 counting steps, so □ = 5. Check: 3 + 5 = 8.

3 The unknown can come first
Find the missing number: □ + 4 = 9.
Show solution Hide solution

Think: what number plus 4 makes 9? Since 5 + 4 = 9, the missing number is 5.

4 Use a make-10 fact
Find the missing number: 6 + □ = 10.
Show solution Hide solution

Think of the two parts that make 10. Six needs 4 more to make 10, so □ = 4.

5 Count on from a known part
Find the missing number: □ + 7 = 12.
Show solution Hide solution

Count on from 7 to 12: 8, 9, 10, 11, 12. That is 5 more, so □ = 5.

6 Use the addition-subtraction connection
Find the missing number: 8 + □ = 15.
Show solution Hide solution

Ask: 15 minus 8 equals what? 15 − 8 = 7, so □ = 7. Check: 8 + 7 = 15.

7 Find a larger missing part
Find the missing number: □ + 6 = 14.
Show solution Hide solution

The whole is 14 and one part is 6. Find the other part: 14 − 6 = 8. So □ = 8.

8 The result can be unknown
Find the missing number: 7 + 5 = □.
Show solution Hide solution

Add: 7 + 5 = 12. Therefore □ = 12.

9 Find an unknown difference
Find the missing number: 9 − 4 = □.
Show solution Hide solution

Subtract 4 from 9: 9 − 4 = 5. So □ = 5.

10 Find a missing subtrahend
Find the missing number: 7 − □ = 3.
Show solution Hide solution

Ask: what must be taken from 7 to leave 3? Since 7 − 4 = 3, □ = 4.

11 Think addition to solve subtraction
Find the missing number: 10 − □ = 6.
Show solution Hide solution

Think: 6 plus what makes 10? Since 6 + 4 = 10, the missing number is 4.

12 Use the whole and the difference
Find the missing number: 12 − □ = 5.
Show solution Hide solution

Compare 12 and 5 to find how many were removed. Since 12 − 5 = 7, □ = 7. Check: 12 − 7 = 5.

13 Check with the related addition fact
Find the missing number: 15 − □ = 8.
Show solution Hide solution

Think: 8 + □ = 15. Eight plus 7 equals 15, so □ = 7.

14 The starting number can be unknown
Find the missing number: □ − 3 = 4.
Show solution Hide solution

Ask: what number gives 4 after 3 is taken away? Add the difference and the amount taken away: 4 + 3 = 7. So □ = 7.

15 Build the unknown whole
Find the missing number: □ − 5 = 6.
Show solution Hide solution

The unknown starting number is made from 6 and 5: 6 + 5 = 11. So □ = 11. Check: 11 − 5 = 6.

16 Solve a subtraction equation within 20
Find the missing number: □ − 8 = 7.
Show solution Hide solution

Add 8 and 7 to rebuild the starting number: 8 + 7 = 15. Therefore □ = 15.

17 The total can be on the left
Find the missing number: 9 = 4 + □.
Show solution Hide solution

Both sides must have the same value. Since 4 + 5 = 9, □ = 5.

18 The subtraction expression can be on the right
Find the missing number: 6 = □ − 2.
Show solution Hide solution

The right side must equal 6. Add 2 back: 6 + 2 = 8. So □ = 8.

19 Find a missing addend when the sum is on the left
Find the missing number: 11 = □ + 3.
Show solution Hide solution

The right side must make 11. Since 11 − 3 = 8, □ = 8.

20 Use equality to check both sides
Find the missing number: 7 = 13 − □.
Show solution Hide solution

We need 13 minus the unknown to equal 7. Since 13 − 6 = 7, □ = 6.

21 Story problem: an unknown part
Mia has 5 stickers. She wants 12 stickers altogether. How many more stickers does she need? 5 + □ = 12.
Show solution Hide solution

The unknown is the missing part. Find 12 − 5 = 7. Mia needs 7 more stickers. Check: 5 + 7 = 12.

22 Story problem: an unknown amount removed
There are 14 birds. Some fly away, and 9 birds remain. How many flew away? 14 − □ = 9.
Show solution Hide solution

The unknown is the amount that left. Find 14 − 9 = 5. So 5 birds flew away. Check: 14 − 5 = 9.

23 Extension: expressions on both sides
Find the missing number: 3 + □ = 5 + 4.
Show solution Hide solution

First find the value of the right side: 5 + 4 = 9. Now solve 3 + □ = 9. Since 3 + 6 = 9, □ = 6.

24 Extension: the unknown comes first
Find the missing number: □ + 2 = 6 + 3.
Show solution Hide solution

The right side is 9. Now solve □ + 2 = 9. Since 7 + 2 = 9, □ = 7.

25 Extension: addition and subtraction together
Find the missing number: 12 − □ = 4 + 3.
Show solution Hide solution

The right side is 4 + 3 = 7. Now solve 12 − □ = 7. Since 12 − 5 = 7, □ = 5.

๐ŸŽฎ Independent Practice — 30 Problems

  1. Find the missing number: 2 + 5 = □.
  2. Find the missing number: 3 + □ = 9.
  3. Find the missing number: □ + 4 = 11.
  4. Find the missing number: 6 + □ = 13.
  5. Find the missing number: □ + 7 = 15.
  6. Find the missing number: 8 + □ = 14.
  7. Find the missing number: □ + 9 = 17.
  8. Find the missing number: 5 + 6 = □.
  9. Find the missing number: 8 − 3 = □.
  10. Find the missing number: 11 − □ = 7.
  11. Find the missing number: 13 − □ = 6.
  12. Find the missing number: 16 − □ = 9.
  13. Find the missing number: □ − 2 = 5.
  14. Find the missing number: □ − 4 = 8.
  15. Find the missing number: □ − 7 = 6.
  16. Find the missing number: 15 − 6 = □.
  17. Find the missing number: 10 = 6 + □.
  18. Find the missing number: 7 = □ + 2.
  19. Find the missing number: 8 = 11 − □.
  20. Find the missing number: 6 = □ − 3.
  21. Find the missing number: 12 = 5 + □.
  22. Find the missing number: 9 = 15 − □.
  23. Extension: find the missing number: 4 + □ = 6 + 2.
  24. Extension: find the missing number: □ + 3 = 5 + 6.
  25. Extension: find the missing number: 14 − □ = 4 + 4.
  26. Extension: find the missing number: 6 + □ = 12 − 2.
  27. Four red balloons are on a table. There are 10 balloons altogether. How many balloons are not red? Write and solve 4 + □ = 10.
  28. Fifteen blocks are in a tower. Some are removed, and 8 remain. How many blocks were removed? Solve 15 − □ = 8.
  29. Solve 7 + □ = 13. Then name one strategy you could use.
  30. Write a subtraction equation in which the missing number is 5.

๐Ÿ† Lesson Test — 15 Questions

  1. Find the missing number: 5 + 4 = □.
  2. Find the missing number: 4 + □ = 11.
  3. Find the missing number: □ + 5 = 13.
  4. Find the missing number: 7 + □ = 16.
  5. Find the missing number: 10 − 6 = □.
  6. Find the missing number: 9 − □ = 2.
  7. Find the missing number: 14 − □ = 10.
  8. Find the missing number: □ − 4 = 7.
  9. Find the missing number: 12 = 7 + □.
  10. Find the missing number: 5 = 9 − □.
  11. Find the missing number: 8 = □ − 5.
  12. Extension: find the missing number: 6 + □ = 5 + 5.
  13. Extension: find the missing number: □ + 4 = 7 + 5.
  14. Thirteen crayons are in a box. Some are given away, and 6 remain. How many crayons were given away? Solve 13 − □ = 6.
  15. Find the missing number in 3 + □ = 11. Then explain how you can check your answer.

๐Ÿ” Complete Answer Key

Show practice answer key Hide practice answer key

Practice Answers

1. 7
2. 6
3. 7
4. 7
5. 8
6. 6
7. 8
8. 11
9. 5
10. 4
11. 7
12. 7
13. 7
14. 12
15. 13
16. 9
17. 4
18. 5
19. 3
20. 9
21. 7
22. 6
23. 4
24. 8
25. 6
26. 4
27. 6 balloons
28. 7 blocks
29. 6. Possible strategy: count on from 7 to 13, or use 13 − 7.
30. Possible answer: 9 − □ = 4, where □ = 5.
Show test answer key Hide test answer key

Test Answers

1. 9
2. 7
3. 8
4. 9
5. 4
6. 7
7. 4
8. 11
9. 5
10. 4
11. 13
12. 4
13. 8
14. 7 crayons
15. 8. Check: 3 + 8 = 11.

๐Ÿ’ก Common Mistakes to Watch For

Thinking the blank is always the result
The unknown can appear before or after the equal sign and in different positions in the equation.
Using the wrong operation
For 7 + □ = 12, subtraction can help: 12 − 7 = 5.
Ignoring the equal sign
Your number must make the values on both sides equal.
Forgetting to check
Replace the box with your answer and read the whole equation again.
What comes next: Lesson 15 uses the addition strategies you already know to add three numbers within 20.

๐Ÿ”— Related Lessons

Lesson 12 — Fact Families: Addition and Subtraction Review how addition and subtraction facts are connected. Lesson 13 — Equal Sign & True/False Equations Review what the equal sign means and how to decide whether an equation is true.

๐Ÿ–จ️ Printable Companion

A future Missing Numbers & Unknowns Practice Pack can include missing-addend cards, part-part-whole models, mixed addition and subtraction equations, equation-balancing challenges, and a complete answer key.

← Lesson 13: Equal Sign & True/False Equations Grade 1 Math Hub Next: Add Three Numbers Within 20 →

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