My 10-year-old 5th grade son is working

Answer

I'll check the working folder for the worksheet, then verify the two items you've described with the skill's math check.

Verified both items against the worksheet script (with the answer withheld):

| # | Problem | His answer | Status | Where it goes wrong |
|---|---|---|---|---|
| 3 | 6.2 + 0.47 | 1.09 | mismatch | ones, tenths, hundredths — every column |
| 6 | 5.4 − 1.28 | 4.14 | mismatch | hundredths column |

Only the two problems you described were transcribed. The other 8 on the worksheet aren't in front of me, so I can't speak to them — those you'll need to check yourself with the same "do the decimal points line up?" question.

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## 1. The misconceptionIt's called **place-value misalignment** — specifically the *decimal-point-right-alignment* error (sometimes listed as "treats decimals like whole numbers" or "right-aligns last digits, not decimal points"). The diagnosis card in this skill calls it **A3**.

Why5th graders fall for it:
- They've spent **two solid years** drilling the whole-number vertical algorithm, where "line up the rightmost digits" is exactly the right rule. That muscle memory transfers to decimals by default.
- The decimal point looks like just another dot on the page — not an anchor. So when they write the problem down, they don't even see the dot as the thing that has to line up.
- They haven't tied "tenths" and "hundredths" to a concrete model (money, base-10 blocks, a place-value chart). The words are memorized; the place-value intuition isn't.
- Worksheets usually don't pre-print the decimal grid, so there's nothing on the page forcing the alignment.

The result is what you saw: he lined up the last digit (the 2 and the 7), did integer arithmetic, and then either dropped a decimal in or copied the dot's position from one of the addends. The number he computed is internally consistent with the *wrong* setup — that's why it "felt right" to him.

## 2. The diagnostic question (one)

**Ask him this, before he touches the page:**

> "About how big should the answer be? Six-point-two plus a little less than one — is the answer closer to 7, or closer to 2? And how do you know?"

**How you'll know it landed:** He says "closer to 7," and gives a reason — either "because 6 plus something is bigger than 6" or "because 6.2 is already more than 6." The moment he says that, the **1.09** answer will look obviously wrong to *him*, not to you. Once he's suspicious of his own answer, you can ask him to look at his paper and find the dot — "where did the dot end up in your answer? Where should it have ended up?" That's where the alignment click happens.

If instead he says "closer to 2," the misconception is deeper — he doesn't yet feel that 6.2 is bigger than 2 — and you'll want to back up to a concrete model (money or a hundreds-grid) before doing any more column arithmetic. But for most 5th graders at this stage, the estimate-first question is enough to surface the error.

## 3. One parallel problem (different numbers, no answer)

**8.3 + 0.19**

Same shape as #3: one decimal place plus two decimal places. The right-alignment trap will produce a wrong answer with the decimal point in the wrong column — exactly the same failure mode. Once he lines the dots up first and then does the column arithmetic, the answer should land just under 8.5, which he can sanity-check against his estimate.

The same fix (line up the dots first, *then* the digits) works for #6 (5.4 − 1.28) — once he sees place value as the anchor instead of the right edge, subtraction fixes itself. No need to give him a separate subtraction practice tonight; one clean addition will recalibrate the rule.

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If, after the estimate question, he still right-aligns and gets a "wrong but plausible-looking" answer, come back and tell me what he wrote and I'll point you to the next rung on the ladder — but don't tell him the right answer, let him find the dot himself.