1st Grade · Number & Operations in Base Ten
1.NBT.C.5: Given a two-digit number, mentally find 10 more or 10 less
1.NBT.C.5 is a 1st 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
Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.
In plain words
Given a two-digit number, mentally find 10 more or 10 less.
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 number is 10 more than 33?
- 43
- 34
- 23
Where students can get stuck
- Counting on by ones, ten separate times, to find 10 more than 56, and often miscounting along the way.
- Changing the wrong digit, saying 10 more than 34 is 35, because adding seems to change the last digit.
- Getting stuck near the ends, like 10 less than 15 or 10 more than 92, where the answer has a different number of digits.
Effective teaching strategies
- Show 56 with ten rods and ones cubes, add one more rod, and ask students to say the new number without counting, noticing that 5 tens became 6 tens.
- List 34, 44, 54, 64 in a column and ask which digit changes and which stays the same, then ask for 10 less than 54.
- Find 10 less than 15 and 10 more than 92 with ten rods, so students see why the answers, 5 and 102, have a different number of digits.
Units that teach this standard
Find where 1.NBT.C.5 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
Everyday Mathematics
i-Ready Classroom Mathematics
Illustrative Mathematics
Math Expressions
Don’t see your curriculum? Contact Bobby and we’ll see if we can help!
Share it with families
Families can read 10 more or 10 less in your head 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.