---
title: "Unknown in Addition & Subtraction"
description: "Determine the unknown whole number in an addition or subtraction equation relating three whole numbers"
canonical: https://lightmysky.com/learn/mathematics/unknown-in-addition-and-subtraction-mt_3JgrHY221M
source: https://lightmysky.com/learn/mathematics/unknown-in-addition-and-subtraction-mt_3JgrHY221M.md
retrieved: 2026-10-19
---

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# Unknown in Addition & Subtraction

Determine the unknown whole number in an addition or subtraction equation relating three whole numbers

Subject: Mathematics · Area: Addition & Subtraction · Ages 6 to 7
Page: https://lightmysky.com/learn/mathematics/unknown-in-addition-and-subtraction-mt_3JgrHY221M

## Ready when they can

- Solve 8 + ? = 11 and write 3
- Solve 5 = □ − 3 and write 8
- Solve 6 + 6 = □ and write 12

## Lesson: Finding the Missing Number

Sometimes an equation has a number hiding. It might look like a question mark or an empty box, like ? or □. Your job is to figure out what number goes there. You already know both sides of an equals sign must match. Use that to find the hidden number.

*(drawing: 8 + ? = 11. Start at 8 and hop forward. You land on 11 after 3 hops, so the missing number is 3.)*

**Example.** Dan had some crayons in a box. He gave 3 away and has 5 left. That means □ − 3 = 5. To undo the take-away, add 3 back: 5 + 3 = 8. Dan started with 8 crayons.

**Tip.** Here is the trick: if a part is missing, use the opposite operation to find it. Missing addend? Subtract. Missing start number in a subtraction? Add. Always check your answer by putting it back into the equation.

**Recap.** To find a missing number, undo the operation you see and keep both sides equal.

## Practice

18 questions on this page, each with its working shown.

## Needs first

- [Inverse: addition undoes subtraction](https://lightmysky.com/learn/mathematics/inverse-addition-undoes-subtraction-mt_ehGS_uVSJv)
- [Addition and subtraction within 20](https://lightmysky.com/learn/mathematics/addition-and-subtraction-within-20-mt_m1W6nTQJ2b)
- [What the equals sign means](https://lightmysky.com/learn/mathematics/what-the-equals-sign-means-mt_oIzycTBeE4)

## Opens up

- [Connecting maths to real life](https://lightmysky.com/learn/mathematics/connecting-maths-to-real-life-mt_f67qGDhyfi)