2nd Grade · Number & Operations in Base Ten
2.NBT.B.8: Mentally add 10 or 100 to a given number 100–900
2.NBT.B.8 is a 2nd grade Common Core math standard in Number & Operations in Base Ten. Here’s what it really covers, and how you can help your students learn it.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
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.
In plain words
Mentally add 10 or 100 to a given number 100–900.
Sample questions from student practice
These questions from Bobby's Math Club are the kinds of questions your students will see. You can try answering them to see what a student might see.
Question 1 of 3
What is 10 less than 100?
- 110
- 90
- 10
Where students can get stuck
- Changing the wrong digit, such as saying 100 more than 432 is 442.
- Getting stuck when the tens digit is 9, answering 2103 or 203 for 10 more than 293 instead of 303.
- Counting on by tens to find 100 more instead of changing the hundreds digit, and losing count along the way.
Effective teaching strategies
- Write 465 in a place-value chart and ask which single digit changes for 10 more and which for 100 more, so students see that the other digits stay put.
- Use rods and flats for numbers where more than one digit changes, such as 10 more than 390 and 10 less than 305, and have students explain each trade.
- Ask students to say the new number without counting up, such as 100 less than 640, and explain which digit changed and why the others did not.
Units that teach this standard
Find where 2.NBT.B.8 lands in the program you teach. Each unit has a guide written for families. Unit names and order vary between editions and districts, so your school’s own materials have the exact sequence.
Bridges in Mathematics
enVision Mathematics
Eureka Math
Eureka Math²
Everyday Mathematics
i-Ready Classroom Mathematics
Illustrative Mathematics
Into Math
Math Expressions
Reveal Math
Don’t see your curriculum? Contact Bobby and we’ll see if we can help!
Share it with families
Families can read Adding or taking away 10 or 100 mentally for a plain-language look at this skill, with three questions to try together and ideas for helping at home.
Help every student practice place value at home
In Bobby's Math Club, students practice at just the right level, with a hint when they need one, a clear explanation after every question, and kid-friendly videos. Unlimited practice is free for every family.
See how it works for teachers
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.