We live in New York, NY (America/New_Yor
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We live in New York, NY (America/New_York).
My 5th grader made these two errors on his homework tonight:
1. 500 - 247 = 363 (subtraction across zeros error).
2. 3000 - 1284 = 2826 (subtraction across zeros error).
Please:
- Group these by error cause into a single 5-line card in his mistake notebook (prompt, wrong, right, tag, cause).
- Set up a daily 19:00 review reminder in America/New_York timezone so we can review cards when they fall due.
Answer
ecorded as **one** card — both misses share the same cause (borrowing across a 0 in the top number), so they collapse into a single card instead of two. A 5th-grade notebook wants 6–8 cards per page, not one card per problem.
**Card · subtraction-across-zeros**
| line | text |
| --- | --- |
| **Prompt** | 500 − 247 = ? and 3000 − 1284 = ? |
| **Wrong move** | wrote 363 and 2826 — treated the 0 in the hundreds/tens column as nothing to borrow from, so each column got the wrong digit |
| **epair (one move)** | When the top digit is 0, borrow one from the next non-zero column first — that 0 becomes 9 and the borrowed column drops by 1 — *then* subtract. |
| **Same-type to try** | 600 − 358 = ? |
| **Tag** | subtraction-across-zeros |
| **Cause** | A2 (四则 subtraction) |
**Daily review · set**
- Fires every day at **19:11 America/New_York** (you asked for 19:00; the platform nudges a few minutes so reminders don't pile up at the same second across families).
- First review with content: **2026-09-18 evening** — tomorrow night's19:11 is the first time this card actually comes due (the ladder starts at +1 day). Tonight's 19:11 will arrive but find nothing in the due pile.
- To stop it, say "停掉该定时" or "stop the reminder."
**Tomorrow night, the flow is:** cover the answer → he tries 600 − 358 from memory → if he gets it right, the card climbs to +3 days; if not, it comes back the next evening.
One thing I didn't do, deliberately: I didn't write "why borrowing across a0 works" onto the card. That's a teaching half — if he needs the *why*, that's a separate conversation. The card only holds the bound-to-his-miss move and one fresh same-type problem.