Mentally adding tens to 3-digit numbers
Mentally add and subtract a three-digit number and tens
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Mentally adding tens to 3-digit numbers means practising the idea at the table with counters or paper models and one clear check. We will demo once, then your child tries.
Materials: 20 counters; Paper and pencil. Adult models 12 − 4 by removing counters, then writes the difference. Child solves three subtraction problems with counters and records each difference. Done: three correct differences with counters matching the writing. Stop after the fresh check; keep optional challenge for another day.
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Start practiceMentally adding tens to 3-digit numbers practice
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Learning resources
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Illustrative Mathematics · Practice link — not the full lesson
Making 124
Before you open · source notes
Building 124 with tens and ones supports mentally adding tens into a 3-digit number. Not every written column sum.
Free, no sign-in. A teacher solution is on the same page, so use the student view with your child.
Before you openAsk the learner to add a ten to a 3-digit number and say the new total.
Opening this link does not save learning or assess mastery.
Source, access and review notes
Public Illustrative Mathematics task page. The .html address responds HTTP 308 to this URL. An anonymous GET of this URL on 2026-11-12 returned HTTP 200 with no login or payment prompt. Where a teacher solution is present, it is on the same page as the student task; this check did not find a separate locked student view. An adult can use the student view section and should not read a solution aloud. Advertising was not observed in the fetched HTML text; that is not a browser, device, or ad-free guarantee. This is a paper or discussion task, not an automatically marked interactive game. Opening the link does not save practice or assess mastery. Diagrams on the page use missing or filename-only alt text, so they are not an accessible image equivalent. Accessibility still depends on an adult describing the picture.
Illustrative Mathematics. “Making 124.” Licensed CC BY-NC-SA 4.0. External task; Gakuva does not copy, host, embed, or redistribute it.
CC BY-NC-SA 4.0 — link only
Check understanding
- Calculate 345 + 40 mentally
- Calculate 462 − 30 mentally
- Explain that only the tens digit changes (unless bridging through a hundred)
“Can your child quickly work out '354 + 40' or '628 − 30' in their head — knowing that only the tens digit changes?”
Curriculum record
- Type
- Procedural
- Subject
- Mathematics
- Domain
- Addition & Subtraction
- Age range
- Ages 7–9
Standards