--- title: "Mentally adding hundreds to 3-digit numbers" description: "Mentally add and subtract a three-digit number and hundreds" canonical: https://lightmysky.com/learn/mathematics/mentally-adding-hundreds-to-3-digit-numbers-mt_xPqczp7zPX source: https://lightmysky.com/learn/mathematics/mentally-adding-hundreds-to-3-digit-numbers-mt_xPqczp7zPX.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 hundreds to 3-digit numbers Mentally add and subtract a three-digit number and hundreds Subject: Mathematics · Area: Addition & Subtraction · Ages 7 to 8 Page: https://lightmysky.com/learn/mathematics/mentally-adding-hundreds-to-3-digit-numbers-mt_xPqczp7zPX ## Ready when they can - Calculate 345 + 200 mentally - Calculate 762 − 400 mentally - Explain that only the hundreds digit changes ## Lesson: Adding and subtracting hundreds in your head You already know how to add or take away 10 or 100 from a number. This works the same way with a whole three-digit number. When you add or subtract hundreds, only the hundreds digit changes. The tens digit and the ones digit stay exactly the same. *(drawing: Start at 345. Hop forward 200. You land on 545. The 4 and the 5 never moved.)* **Example.** Ben has 762 marbles. He gives away 400 marbles. The tens digit (6) and the ones digit (2) do not change. Only the hundreds digit changes: 7 hundreds minus 4 hundreds is 3 hundreds. Ben has 362 marbles left. **Tip.** Cover the tens and ones digits with your finger. Add or subtract just the hundreds, like a one-digit number. Then put the tens and ones back where they were. **Recap.** Adding or subtracting hundreds only changes the hundreds digit; the tens and ones stay exactly the same. ## 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)