Number and operations in base ten
1.NBT.C.5
Given a two-digit number, mentally find 10 more or 10 less without having to count.
Practice for this standard
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Counting and number sense
Counting on, skip counting, number recognition and the early number sense that everything else is built on.
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Place value
Tens and ones through millions and thousandths, expanded form, and reading numbers out loud correctly.
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Mental math
Bridging ten, doubles and near doubles, compensation, and other strategies worth naming.
Official wording and the full standard: CCSS.MATH.CONTENT.1.NBT.C.5