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
Teacher's field guide
What This Standard Means
What Students Need to Do
- Students mentally change a three-digit number by one ten or one hundred without counting by ones. They use place value to track which digits change, including across a new ten or hundred.
What Mastery Looks Like
- Students quickly and accurately find each new number without written computation. They explain how the tens or hundreds changed and when an exchange affected another place.
Common Misconceptions
- Students may change the wrong place, append a zero, or count by ones. They may also miss regrouping, such as answering 305 for 395 + 10.
How to Assess It
- Give this exit ticket: Solve mentally: 246 + 10, 395 + 10, 612 - 10, and 700 - 100. Explain which place changed in one problem.
Lesson moves
Ways to Teach It
Build 364 with base-ten blocks, add one ten, then remove one hundred and record each new number.
Ask, “Why does 395 + 10 change more than the tens digit?” Students explain with a place value chart.
Play a card game where students draw a number, spin +10, -10, +100, or -100, and state the answer mentally.
Present a library with 425 books, then add a shipment of 100 books and remove 10 damaged books.
Free download
Printable 2.NBT.B.8 Worksheet
A ready-to-print activity worksheet aligned to CCSS.Math.Content.2.NBT.B.8, with an answer key for the teacher on its own page. No account needed.
PDF, US Letter, prints cleanly in black and white.
Download the worksheetLearning progression
Where This Standard Sits
See the whole path on the Number & Operations in Base Ten progression map.
Before This Standard
If students are struggling here, check these first.
- CCSS.Math.Content.2.NBT.A.1
Students use hundreds, tens, and ones to know which digit changes when adding or subtracting 10 or 100 mentally.
- CCSS.Math.Content.1.NBT.C.5
Students extend changing the tens digit by 10 to three-digit numbers and add changing the hundreds digit by 100.
- CCSS.Math.Content.2.NBT.A.2
Skip-counting by 10s and 100s supports seeing how numbers change when mentally adding or subtracting tens and hundreds.
Keep exploring
Related Standards
- 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...
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