Number and operations in base ten
2.NBT.B.9
Explain why addition and subtraction strategies work.
Practice for this standard
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Addition
Sums within 10 through multi-digit column addition with regrouping, plus the fact fluency underneath.
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Subtraction
Taking away, counting up, and multi-digit subtraction with borrowing across zeros.
Official wording and the full standard: CCSS.MATH.CONTENT.2.NBT.B.9