Mathematics

Addition and Subtraction

The most common mistakes Singapore primary students make in Addition and Subtraction, distilled from 69+ MOE-aligned questions across P1, P2, P3. Free practice at every level.

From the question bank

Top 10 mistakes in Addition and Subtraction

  1. #1Misconception

    30 + 40 ≠ 10. That would be 40 − 30. Add the tens: 3 + 4 = 7, so 30 + 40 = 70.

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  2. #2Misconception

    34 = 30 + 4. But the sum is 30 + 40, not 30 + 4. 70 is correct.

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  3. #3Calc error

    74 = 70 + 4. But 30 + 40 = 70 only. There are no extra ones.

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  4. #4Misconception

    15 is the total after buying. The starting number is 9, and he buys 6: 9 + 6.

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  5. #5Misconception

    Sam already has 9 and gets 6 more. 9 + 6 = 15 is the correct sentence.

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  6. #6Misconception

    It looks like the tens column was skipped. Only the hundreds (2 + 1 = 3) and units (4 + 8 = 12, write 2 carry 1) were added, giving 302. Always work through every column: units, tens, then hundreds.

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  7. #7Calc error

    You added the units correctly (4 + 8 = 12, write 2 carry 1), but forgot to add the carried 1 to the tens column. 3 + 5 = 8, not 9. Always remember to add the carry to the next column.

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  8. #8Calc error

    You correctly carried 1 from the units to the tens (3 + 5 + 1 = 9), but then carried an extra 1 into the hundreds column by mistake. There is no carry from the tens here, so hundreds = 2 + 1 = 3, not 4.

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  9. #9Misconception

    You rounded 39 up to 40 and got 75 − 40 = 35. But then you subtracted 1 again instead of adding it back. When you over-subtract by rounding up, you must add back the difference. 35 + 1 = 36, not 34.

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  10. #10Misconception

    You rounded 39 up to 40 and subtracted correctly: 75 − 40 = 35. But then you added 9 back instead of 1. When you round up by 1, you only add back 1, not the whole rounded amount. 35 + 1 = 36.

    Seen in 1 question

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