--- title: "Mentally adding tens to 3-digit numbers" description: "Mentally add and subtract a three-digit number and tens" canonical: https://lightmysky.com/learn/mathematics/mentally-adding-tens-to-3-digit-numbers-mt_aHQ9kNt3is source: https://lightmysky.com/learn/mathematics/mentally-adding-tens-to-3-digit-numbers-mt_aHQ9kNt3is.md retrieved: 2026-10-19 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Mentally adding tens to 3-digit numbers Mentally add and subtract a three-digit number and tens Subject: Mathematics · Area: Addition & Subtraction · Ages 7 to 8 Page: https://lightmysky.com/learn/mathematics/mentally-adding-tens-to-3-digit-numbers-mt_aHQ9kNt3is ## Ready when they can - Calculate 345 + 40 mentally - Calculate 462 − 30 mentally - Explain that only the tens digit changes (unless bridging through a hundred) ## Lesson: Adding and subtracting tens in your head A three-digit number has a hundreds digit, a tens digit, and a ones digit. When you add or subtract tens, like 20, 30, or 40, you are only changing groups of ten. That means the tens digit is the one that moves. The hundreds and ones digits usually stay put. *(drawing: Start at 345 and hop four jumps of ten to land on 385. Only the tens digit moves, from 4 to 8.)* **Example.** Take 354 + 40. The hundreds digit is 3 and the ones digit is 4, so those stay the same. The tens digit goes up by 4, from 5 to 9. So 354 + 40 = 394. **Tip.** Watch what happens when the tens digit gets big. If adding tens pushes it past 9, the extra spills into the hundreds digit. For example, 380 + 40: 8 tens plus 4 tens is 12 tens, which is 1 hundred and 2 tens. So 380 + 40 = 420, and the hundreds digit changes too. **Example.** Now try 462 − 30. The ones digit (2) and hundreds digit (4) stay the same. The tens digit drops by 3, from 6 to 3. So 462 − 30 = 432. **Recap.** Add or subtract only the tens digit, unless the tens spill past 9 or below 0, then the hundreds digit shifts too. ## 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) - [10 or 100 More or Less](https://lightmysky.com/learn/mathematics/10-or-100-more-or-less-mt_izien3ZX51) ## Opens up - [Mental addition and subtraction (age 9+)](https://lightmysky.com/learn/mathematics/mental-addition-and-subtraction-age-9-mt_MWXPiaTnEu)