CCSS.Math.Content.2.NBT.B.7
The standard
Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method. Understand that in adding or subtracting three-digit numbers, one adds or subtracts hundreds and hundreds, tens and tens, ones and ones; and sometimes it is necessary to compose or decompose tens or hundreds.
Common Core State Standards for Mathematics · Number and Operations in Base Ten
What this standard means
Students need to add and subtract three-digit numbers by thinking about hundreds, tens, and ones. They should use base-ten blocks, drawings, expanded form, open number lines, or equations, then connect that work to a written method.
Mastery looks like explaining why they regroup, not just following steps. Students can show that 243 + 178 means combining ones with ones, tens with tens, and hundreds with hundreds. They often get stuck when a ten or hundred must be made or broken apart, especially with zeros in numbers like 405 or 600.
Ways to teach it
- Hands-on: Give pairs base-ten blocks and have them solve 346 + 287, trading ones for tens and tens for hundreds as needed.
- Prompt: Ask students to explain, with a drawing, why 512 - 268 needs a hundred broken into tens.
- Quick assessment: Have students solve 734 - 459 two ways, then circle where they regrouped.
- Real-world connection: Use a school supply order, such as 248 pencils plus 375 pencils, and ask students to find the total shipped.
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