---
title: "Mentally adding and subtracting ones with 3-digit numbers"
description: "Mentally add and subtract a three-digit number and ones"
canonical: https://lightmysky.com/learn/mathematics/mentally-adding-and-subtracting-ones-with-3-digit-numbers-mt_oDU_8zMZjp
source: https://lightmysky.com/learn/mathematics/mentally-adding-and-subtracting-ones-with-3-digit-numbers-mt_oDU_8zMZjp.md
retrieved: 2026-09-12
---

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# Mentally adding and subtracting ones with 3-digit numbers

Mentally add and subtract a three-digit number and ones

Subject: Mathematics · Area: Addition & Subtraction · Ages 7 to 8
Page: https://lightmysky.com/learn/mathematics/mentally-adding-and-subtracting-ones-with-3-digit-numbers-mt_oDU_8zMZjp

## Ready when they can

- Calculate 345 + 7 mentally
- Calculate 462 − 5 mentally
- Explain that only the ones digit changes (unless bridging through a ten)

## Lesson: Adding and subtracting ones from a big number

A three-digit number is built from hundreds, tens, and ones. When you add or take away a small number, you are really only working with the ones. Most of the time, the hundreds and tens stay exactly the same.

**Example.** Take 341 + 6. The ones digit is 1, and 1 + 6 = 7. The hundreds and tens do not change at all, so 341 + 6 = 347.

*(drawing: 5 + 7 = 12. The extra ten carries into the tens place, so 345 + 7 = 352.)*

**Example.** Now look at 462 - 5. The ones digit is 2, and you cannot take 5 away from 2. Borrow a ten from the tens place: 12 - 5 = 7, and the tens digit drops from 6 to 5. So 462 - 5 = 457.

**Tip.** To check if the tens will change, look only at the ones digit. If adding it makes 10 or more, or subtracting it goes below zero, the tens digit changes too.

**Recap.** Check the ones digit first: the tens only change when adding or subtracting crosses ten.

## Practice

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

## Needs first

- [The three digits of a three-digit number](https://lightmysky.com/learn/mathematics/the-three-digits-of-a-three-digit-number-mt_aPBzD28_mT)
- [Fluent adding and subtracting within 100](https://lightmysky.com/learn/mathematics/fluent-adding-and-subtracting-within-100-mt_cChv2j_-Da)
- [Mental and written addition and subtraction](https://lightmysky.com/learn/mathematics/mental-and-written-addition-and-subtraction-mt_wzAZ8qFDc4)

## Opens up

- [Mental addition and subtraction (age 9+)](https://lightmysky.com/learn/mathematics/mental-addition-and-subtraction-age-9-mt_MWXPiaTnEu)