My 5th-grade son is practicing decimal a
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My 5th-grade son is practicing decimal addition and made the exact same alignment error across three problems on his worksheet:
1. 3.5 + 0.48 = 0.83 (he aligned 5 over 8 and 3 over 4 like whole numbers: 35 + 48 = 83)
2. 12.3 + 1.45 = 1.58 (he aligned the rightmost digits 3 over 5, 2 over 4)
3. 7.2 + 0.19 = 0.91 (he treated it as 72 + 19 = 91)
He keeps right-aligning decimals instead of lining up the decimal points.
Can you cluster these three missed problems into a single 5-line cause card (prompt, wrong, right, tag, cause) for our mistake notebook, and give one parallel problem for him to test his alignment tonight without lecturing on general math theory?
Answer
Card saved as one cluster (id `513d13bbb367fd50`) — five lines, no rule-chart attached:
| line | what was written |
|---|---|
| prompt | `3.5 + 0.48 = ?` (same cause also on `12.3 + 1.45` and `7.2 + 0.19` — three problems, one card) |
| wrong | wrote `0.83` — treated like whole numbers `35 + 48`, right-aligned the rightmost digits (5 over 8, 3 over 4) |
| right | `8.3 + 0.19` → line up the decimal points first, tenths over tenths → `8.49` |
| tag | `decimal-addition-alignment` |
| cause | `A3` (decimal point right-aligned instead of place-value aligned) |
The cluster holds three problems under one cause, as you asked — one card per Denkfehler, not one card per question.
**Tonight's parallel** (book closed, no notes in front):
> `8.3 + 0.19 = ?`
What does he write? If the answer is `8.49`, the alignment fix is sitting with him. If he slides back into right-aligning the last digits and lands at `8.3 + 0.19 = 0.49` or similar, A3 is the active pattern and the card will keep coming back on the `[1, 3, 7, 14, 30]`-day ladder.
**Card schedule:** first spaced review lands **tomorrow, 2026-09-18** (day +1). The same ladder means the next due after a `pass` moves to day +3, then +7, +14, +30.
I did not give a general rule on why decimal alignment works — that half is homework-tutor's lane; this notebook only stores the wrong move, the one same-type repair, the tag, and the cause.