CCSS.Math.Content.1.NBT.B.2c
The standard
The numbers 10, 20, 30, 40, 50, 60, 70, 80, 90 refer to one, two, three, four, five, six, seven, eight, or nine tens (and 0 ones).
Common Core State Standards for Mathematics
What this standard means
Students need to see 10, 20, 30, and so on as groups of ten, not just numbers to count by. They should connect each decade number to that many tens and zero leftover ones, using objects, drawings, words, and numerals.
Mastery looks like a student saying, “60 is six tens and no ones,” then building it with six ten-sticks or bundles. Students often get stuck counting individual ones, mixing up the number of tens with the digit name, or thinking the zero means nothing instead of zero ones.
Ways to teach it
- Hands-on activity: Give students ten-sticks and have them build 30, 50, and 80, then label each with tens and ones cards.
- Discussion prompt: Ask, “How is 70 different from 7?” and have students explain using a drawing or base-ten blocks.
- Quick assessment: Show 40, 90, and 20, and ask students to write how many tens and how many ones for each.
- Real-world connection: Use stacks of ten cups or bundled pencils to show how 10, 20, and 30 are counted in groups.
Plan a lesson for CCSS.Math.Content.1.NBT.B.2c
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Related standards
- CCSS.Math.Content.1.NBT.B.2a
10 can be thought of as a bundle of ten ones — called a "ten."
- CCSS.Math.Content.2.NBT.A.1b
The numbers 100, 200, 300, 400, 500, 600, 700, 800, 900 refer to one, two, three, four, five, six, seven, eight, or nine hundreds (and 0 tens and 0 ones).
- CCSS.Math.Content.1.NBT.B.2b
The numbers from 11 to 19 are composed of a ten and one, two, three, four, five, six, seven, eight, or nine ones.
- CCSS.Math.Content.3.NBT.A.3
Multiply one-digit whole numbers by multiples of 10 in the range 10—90 (e.g., 9 × 80, 5 × 60) using strategies based on place value and properties of operations...