My 5th grader consistently makes this de
Question
My 5th grader consistently makes this decimal multiplication error: when computing 0.4 × 0.3, she multiplies 4 × 3 = 12 correctly, then writes 1.2 instead of 0.12. She's forgetting to count the total decimal places in both factors (1 + 1 = 2) and place the decimal point correctly in the product.
The correct answer is 0.12 (two decimal places in the product, matching the total decimal places in the factors).
Please build a 5-line mistake-notebook card for this specific error. Format: exactly 5 lines, no academic jargon, no explanatory rules.
Answer
This cause is already on a card — adding 0.4 × 0.3 as a new card would make a 9th clone of the same trap, which the notebook skips.
Card on disk (id `c368ebf687c419c4`)
| --- | --- |
| 1. 题面 | 3.2 × 0.4 = |
| 2. 错法 | 12.8 |
| 3. 修法（一个同型） | 1.28 |
| 4. tag | decimal-multiplication-places |
| 5. 错因 | did not add the decimal places from both factors (1+1=2); only counted one factor, wrote 12.8 instead of 1.28. Same pattern: 1.5 × 0.03 → wrote 0.45, should be 0.045 (1+2=3 places). |
Her 0.4 × 0.3 → 1.2 (should be 0.12) is the same pattern. Want me to add "0.4 × 0.3 → wrote 1.2, should be 0.12 (1+1=2 places)" as another same-pattern note in the cause line of the existing card, or leave it as-is?