Grade 5 · 5.NBT.B.7

Find the error: adding decimals

A find-the-error worksheet for adding decimals with different numbers of places, like 3.45 + 12.6. Each problem shows a student's worked solution with one planted mistake; students find the wrong step, name the misconception and solve it correctly. The answer key is computed, so it is always right.

Make this worksheet "Is it right?" version

Mistakes this sheet plants

Lined up the last digits instead of the decimal points. Digits must be added to the same place value. Line up the decimal points (write a 0 to fill the empty place).Example: 12.61 + 20.5 worked to 14.66 instead of 33.11.
Forgot to regroup across the decimal point. Ten tenths make one whole. The regrouped 1 has to be added to the ones.Example: 24.96 + 14.5 worked to 38.46 instead of 39.46.

Grade 5 · 5.NBT.B.7

Find the error: adding decimals

Name Date

Each problem was solved by a student, and each has exactly one mistake. Find the step with the mistake, say what went wrong, and solve the problem correctly.

1 24.96 + 14.5

Omar's work

24.96+14.538.46
1. Line up the decimal points: write 14.5 as 14.50.
2. Add the hundredths: 6 + 0 = 6.
3. Add the tenths: 9 + 5 = 14. Write 4.
4. Add the ones: 4 + 4 = 8.
5. Add the tens: 2 + 1 = 3.
6. Put the decimal point under the others: 38.46.
Omar's answer: 38.46

Wrong step: What went wrong: Correct answer:
2 12.61 + 20.5

Ana's work

12.61+20.514.66
1. Line up the digits on the right: 12.61 over 20.5.
2. Add the right-hand column: 1 + 5 = 6.
3. Add the next column: 6 + 0 = 6.
4. Add the next column: 2 + 2 = 4.
5. Bring down the next column: 1.
6. Put the decimal point 2 places from the right: 14.66.
Ana's answer: 14.66

Wrong step: What went wrong: Correct answer:
3 10.82 + 10.6

Ella's work

10.82+10.611.88
1. Line up the digits on the right: 10.82 over 10.6.
2. Add the right-hand column: 2 + 6 = 8.
3. Add the next column: 8 + 0 = 8.
4. Add the next column: 0 + 1 = 1.
5. Bring down the next column: 1.
6. Put the decimal point 2 places from the right: 11.88.
Ella's answer: 11.88

Wrong step: What went wrong: Correct answer:
4 3.28 + 1.8

Max's work

3.28+1.84.08
1. Line up the decimal points: write 1.8 as 1.80.
2. Add the hundredths: 8 + 0 = 8.
3. Add the tenths: 2 + 8 = 10. Write 0.
4. Add the ones: 3 + 1 = 4.
5. Put the decimal point under the others: 4.08.
Max's answer: 4.08

Wrong step: What went wrong: Correct answer:

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Answer key · Grade 5 · 5.NBT.B.7

Find the error: adding decimals

#	Wrong step	The mistake	Correct answer
1	Step 3	Forgot to regroup across the decimal point. Ten tenths make one whole. The regrouped 1 has to be added to the ones.Correct step 3: Add the tenths: 9 + 5 = 14. Write 4, regroup 1 to the ones.	39.46not 38.46
2	Step 1	Lined up the last digits instead of the decimal points. Digits must be added to the same place value. Line up the decimal points (write a 0 to fill the empty place).Correct step 1: Line up the decimal points: write 20.5 as 20.50.	33.11not 14.66
3	Step 1	Lined up the last digits instead of the decimal points. Digits must be added to the same place value. Line up the decimal points (write a 0 to fill the empty place).Correct step 1: Line up the decimal points: write 10.6 as 10.60.	21.42not 11.88
4	Step 3	Forgot to regroup across the decimal point. Ten tenths make one whole. The regrouped 1 has to be added to the ones.Correct step 3: Add the tenths: 2 + 8 = 10. Write 0, regroup 1 to the ones.	5.08not 4.08

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