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
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students solve two-digit addition and subtraction problems, then explain why their strategies work. They connect each step to tens, ones, regrouping, and properties such as commutativity or associativity.
What Mastery Looks Like
- Students solve addition and subtraction problems accurately using place value strategies. They use equations, drawings, or base-ten blocks to explain each step and connect it to a property of operations.
Common Misconceptions
- Students may describe steps without explaining why they work. They may regroup digits without connecting the trade to ten ones equaling one ten, or treat subtraction as commutative.
How to Assess It
- Give students 46 + 27 and ask, “Solve it two ways, then explain why one strategy works using tens and ones.”
Lesson moves
Ways to Teach It
Build 38 + 25 with base-ten blocks, trade ten ones for one ten, and label each step with an equation.
Ask students to explain why breaking 54 into 50 and 4 helps when solving 54 minus 28.
Play Strategy Match by pairing problem cards with solution cards showing expanded form, compensation, or regrouping, then justify each match.
Plan classroom supply orders with two-digit quantities, then explain how place value makes finding totals and differences easier.
Free download
Printable 2.NBT.B.9 Worksheet
A ready-to-print activity worksheet aligned to CCSS.Math.Content.2.NBT.B.9, with an answer key for the teacher on its own page. No account needed.
PDF, US Letter, prints cleanly in black and white.
Download the worksheetLearning progression
Where This Standard Sits
See the whole path on the Number & Operations in Base Ten progression map.
Before This Standard
If students are struggling here, check these first.
- CCSS.Math.Content.1.OA.B.3
Using commutative and associative properties for basic operations supports explaining why multi-digit addition and subtraction strategies work.
- CCSS.Math.Content.1.NBT.B.2
Explaining two-digit addition and subtraction strategies requires seeing numbers as tens and ones and tracking how those units combine or separate.
- CCSS.Math.Content.2.NBT.B.5
Using place-value and operation strategies gives students concrete methods to justify when explaining why addition and subtraction work.
Keep exploring
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.2.NBT.B
Use place value understanding and properties of operations to add and subtract.
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