Unknown in Addition & Subtraction
Determine the unknown whole number in an addition or subtraction equation relating three whole numbers
Lesson
Set out your lesson
“Unknown in Addition & Subtraction means finding the missing whole number in a three-number addition or subtraction equation. We will build □ with counters, solve one addition and one subtraction unknown, then check a fresh equation.”
Bring to the table
- 12 counters or blocks
- Paper equations with a box for the unknown (□)
- Optional: adult-previewed Illustrative Mathematics companion task after the table demo
Extra for the adult
Worked example. In 3 + □ = 7, the unknown is 4 because 3 and 4 make 7.
Optional challenge. For Unknown in Addition & Subtraction, place the unknown in the first position (□ − 3 = 5) and explain.
What to notice. Keep one parent note: equations; unknowns; strategies; support used. This session samples unknowns, not mastery of every equation.
Share and support
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Teaching approaches
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1 approach
Unknown in Addition & Subtraction means finding the missing whole number in a three-number addition or subtraction equation. We will build □ with counters, solve one addition and one subtraction unknown, then check a fresh equation.
Spend about 12 minutes finding unknowns with counters and equations, then checking a new □ problem.
Family records and proof options
Keep a record of your learning
Save photos, notes, and attendance. Export a proof packet for compliance or portfolio reviews.
Practice this concept
Start practiceUnknown in addition & subtraction practice
Find the missing number in three-number equations.
Free · no login · no ads · Practice answers and completion are not saved. Parent observation remains the lesson record.
Learning resources
Extra practice links — they do not replace Gakuva’s teaching approach.
Illustrative Mathematics · Practice link — not the full lesson
Find the Missing Number
Before you open · source notes
Six written addition and subtraction equations put the missing value in different positions. Answers are on the teacher portion of the same page. Use the student view while teaching.
Free, no sign-in. A teacher solution is on the same page, so use the student view with your child.
Before you openThis is a written task. Keep the solution private from the learner. The page does not mark the equations automatically.
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 2026-11-10 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. “Find the Missing Number.” 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
- Solve 8 + ? = 11 and write 3
- Solve 5 = □ − 3 and write 8
- Solve 6 + 6 = □ and write 12
“If your child sees '? + 6 = 14' or '9 − ? = 4' written on paper, can they figure out what number goes in the gap?”
Curriculum record
- Type
- Procedural
- Subject
- Mathematics
- Domain
- Addition & Subtraction
- Age range
- Ages 6–11
Standards