CCSS.Math.Content.2.NBT.B.9
The standard
Explain why addition and subtraction strategies work, using place value and the properties of operations.
Common Core State Standards for Mathematics · Number and Operations in Base Ten
What this standard means
Students need to explain the thinking behind addition and subtraction strategies, not just get the answer. They should use tens, ones, making a ten, breaking numbers apart, and properties like changing the order of addends.
Mastery looks like a student solving 47 + 25 and saying, “40 + 20 is 60, 7 + 5 is 12, so 72,” or explaining regrouping with base-ten blocks. Students often get stuck using steps without meaning, especially when they trade ten ones for one ten or break apart numbers in subtraction.
Ways to teach it
- Have students build 36 + 28 with base-ten blocks, combine ones, trade for a ten, then explain each move to a partner.
- Ask students to write: Why does breaking 54 into 50 and 4 help solve 54 minus 27?
- Show two solved problems, one correct and one incorrect, and ask students to circle the mistake and explain it.
- Use a classroom store receipt with two prices, then have students explain how they found the total using tens and ones.
Plan a lesson for CCSS.Math.Content.2.NBT.B.9
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Related standards
- CCSS.Math.Content.1.NBT.C
Use place value understanding and properties of operations to add and subtract.
- CCSS.Math.Content.1.OA.B
Understand and apply properties of operations and the relationship between addition and subtraction.
- CCSS.Math.Content.1.OA.B.3
Apply properties of operations as strategies to add and subtract.
- CCSS.Math.Content.2.NBT.B
Use place value understanding and properties of operations to add and subtract.