Grade 1 · Math · Addition

1st Grade Addition Within 20 Error Detective worksheets

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Addition Within 20 Error Detective

Grade 1 Math Worksheet - Challenge Me

NameDate

Circle the answer, or write the answer on the line.

1.
A student wrote 4 + 16 = 20. Is it correct?
- no
- yes
A student wrote 12 + 6 = 19. That is a mistake. What should it be?
3.
Only one of these addition facts has a mistake. Which one?
- 14 + 5 = 19
- 15 + 2 = 17
- 11 + 3 = 15
- 9 + 2 = 11
4.
A student wrote 5 + 9 = 12. Is it correct?
- no
- yes
A student wrote 8 + 10 = 20. That is a mistake. What should it be?
6.
Only one of these addition facts has a mistake. Which one?
- 17 + 3 = 20
- 14 + 3 = 18
- 4 + 12 = 16
- 7 + 11 = 18
7.
A student wrote 17 + 2 = 19. Is it correct?
- no
- yes
A student wrote 8 + 6 = 16. That is a mistake. What should it be?
9.
Only one of these addition facts has a mistake. Which one?
- 16 + 4 = 20
- 15 + 3 = 18
- 5 + 7 = 12
- 11 + 5 = 15
10.
A student wrote 12 + 4 = 14. Is it correct?
- no
- yes

1st Grade Addition Within 20 Error Detective worksheet 1 – printable PDF — Download PDF: 1 2 3 4 5 6

About this worksheet

Addition Within 20 Error Detective gives grade 1 students a focused page for a skill that parents and teachers often need as its own printable worksheet. Error detective pages build fluency and reasoning, a teacher-utility format missing from broad fact practice. First graders may spot wrong answers but need practice replacing them with the correct sum. The items rotate through several question types so the page is not just the same prompt with different numbers or words. Students check a shown sum, decide whether it is correct, and write or choose the correction.

Support versions keep the reading load, numbers, or choices friendlier while preserving the same thinking. Standard versions match everyday classroom practice. Challenge versions add a closer distractor, an extra reasoning step, or less familiar examples. Before answering, students should say what the question is asking, point to the evidence, and check that the answer fits the exact skill named in the title.

What your child will learn

Check addition facts
Find one-off errors
Explain a correction

Common Core standard: 1.OA.C.6

Worked example

If a student says 8 + 5 = 12, count on from 8: 9, 10, 11, 12, 13. The correct sum is 13.

Common mistake

Marking an answer correct because it is close to the real sum.

Helpful hint

Count on from the greater addend.

Questions parents ask

What is the most common mistake with addition within 20 error detective?

Marking an answer correct because it is close to the real sum.

How can I help my child with addition within 20 error detective?

Count on from the greater addend.

Are these 1st grade math worksheets free to print?

Yes. All 6 printable versions on this page are free to download and print, with no account needed.

More 1st grade addition

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