CCSS.Math.Content.1.NBT.C.4
The standard
Add within 100, including adding a two-digit number and a one-digit number, and adding a two-digit number and a multiple of 10, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between addition and subtraction; relate the strategy to a written method and explain the reasoning used. Understand that in adding two-digit numbers, one adds tens and tens, ones and ones; and sometimes it is necessary to compose a ten.
Common Core State Standards for Mathematics · Number and Operations in Base Ten
What this standard means
Students need to add within 100 by thinking in tens and ones, not by memorizing steps. They should use cubes, ten-sticks, drawings, open number lines, or place-value charts to show what is happening when they add a two-digit number to a one-digit number or to a multiple of 10.
Mastery looks like a student saying, “34 plus 20 is 54 because I added 2 tens,” or “47 plus 8 makes 55 because 7 ones and 8 ones make 15 ones.” Common stuck points are mixing tens and ones, forgetting to make a new ten, and writing answers without explaining the model or strategy.
Ways to teach it
- Hands-on activity: Give students ten-sticks and ones cubes to build 36 plus 7, trade 10 ones for a ten, then record the equation.
- Prompt: Ask, “How is 42 plus 30 different from 42 plus 3?” and have students explain with a drawing.
- Quick assessment: Show 58 plus 6 and ask students to solve it two ways, using a model and a written equation.
- Real-world connection: Use a class store with 10-cent rods and 1-cent cubes to add prices like 24 cents plus 30 cents.
Plan a lesson for CCSS.Math.Content.1.NBT.C.4
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