CCSS.Math.Content.2.NBT.B.8
The standard
Mentally add 10 or 100 to a given number 100—900, and mentally subtract 10 or 100 from a given number 100—900.
Common Core State Standards for Mathematics · Number and Operations in Base Ten
What this standard means
Students need to change a three-digit number by one ten or one hundred without counting by ones. They should know that adding 10 changes the tens place, adding 100 changes the hundreds place, and subtraction works the same way when no regrouping is needed.
Mastery looks like quick, accurate mental answers and a short explanation like, “346 plus 100 is 446 because the hundreds went up by 1.” Students often get stuck when crossing a hundred, like 390 plus 10, or when they change the wrong digit in numbers with zeros, like 407 minus 100.
Ways to teach it
- Hands-on activity: Give students base-ten blocks and have them trade cards such as 235 plus 10, 608 minus 100, and 390 plus 10.
- Prompt: Write 472 on the board and ask, “What changes when I add 10, and what stays the same?”
- Quick assessment: Call out five numbers from 100 to 900, and students write plus 10, minus 10, plus 100, and minus 100 answers.
- Real-world connection: Use pretend bank deposits and withdrawals of $10 and $100, starting with amounts like $240, $510, and $890.
Plan a lesson for CCSS.Math.Content.2.NBT.B.8
Generate a complete lesson plan aligned to this standard, with objectives, activities, and materials. Free, no account needed.
Related standards
- CCSS.Math.Content.1.NBT.C.5
Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
- CCSS.Math.Content.2.NBT.B.7
Add and subtract within 1000, using concrete models or drawings and strategies based on place value, properties of operations, and/or the relationship between a...
- 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...
- CCSS.Math.Content.1.NBT.C.6
Subtract multiples of 10 in the range 10-90 from multiples of 10 in the range 10-90 (positive or zero differences), using concrete models or drawings and strate...