---
title: "Finding a missing number in addition"
description: "Understand subtraction as finding an unknown addend (e.g. 10 − 8 = ? is the same as 8 + ? = 10)"
canonical: https://lightmysky.com/learn/mathematics/finding-a-missing-number-in-addition-mt_ezc2m_0dzN
source: https://lightmysky.com/learn/mathematics/finding-a-missing-number-in-addition-mt_ezc2m_0dzN.md
retrieved: 2026-10-19
---

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# Finding a missing number in addition

Understand subtraction as finding an unknown addend (e.g. 10 − 8 = ? is the same as 8 + ? = 10)

Subject: Mathematics · Area: Addition & Subtraction · Ages 6 to 7
Page: https://lightmysky.com/learn/mathematics/finding-a-missing-number-in-addition-mt_ezc2m_0dzN

## Ready when they can

- Solve 10 − 8 by thinking 'what do I add to 8 to make 10?'
- Explain that subtraction can be thought of as a missing-addend problem
- Use known addition facts to solve related subtraction problems

## Lesson: Finding the missing number

When you see a subtraction problem like 10 − 8 = ?, you can turn it into an addition question instead. Ask yourself: what number do I add to 8 to reach 10?

*(drawing: Start at 8 and hop to 10. You hopped 2 spaces, so 8 + 2 = 10 and 10 − 8 = 2.)*

**Example.** Mia has 8 crayons. She wants 10 crayons. She thinks: 8 + ? = 10. She counts up 2 more, so she needs 2 more crayons. That means 10 − 8 = 2.

**Tip.** You already know your addition facts. Use them backwards! If you know 6 + 4 = 10, then you also know 10 − 6 = 4 and 10 − 4 = 6.

**Recap.** 10 − 8 = ? is the same question as 8 + ? = 10, so your addition facts can solve subtraction too.

## Practice

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

## Needs first

- [Addition as combining or putting together two groups](https://lightmysky.com/learn/mathematics/addition-as-combining-or-putting-together-two-groups-mt_OvyoRo47K-)
- [Subtraction as taking away or separating](https://lightmysky.com/learn/mathematics/subtraction-as-taking-away-or-separating-mt_zuKAX6lcYR)

## Opens up

- [Inverse: addition undoes subtraction](https://lightmysky.com/learn/mathematics/inverse-addition-undoes-subtraction-mt_ehGS_uVSJv)