1st Grade · Number & Operations in Base Ten
1.NBT.C.4: Add within 100
1.NBT.C.4 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
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.
In plain words
Add within 100.
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
If you have 2 tens and 4 ones, and then you get 5 more ones, how many do you have in total?
- 30
- 29
- 28
Where students can get stuck
- Lining the 7 up under the 3 in 36 + 7, adding it to the tens, and getting 106 instead of 43.
- Writing down all 13 ones instead of making a new ten, so 36 + 7 becomes 313 or 3013.
- Treating 36 + 20 like 36 + 2 and answering 38, because the zero in 20 seems unimportant.
Effective teaching strategies
- Use ten rods and ones cubes for 36 + 7, keeping the 7 beside the 6 ones, so students see ones are added to ones and not to the tens.
- Have students trade 10 of the 13 ones for a new ten rod, then write the result as 4 tens and 3 ones, which is 43.
- Compare 36 + 2 and 36 + 20 side by side with blocks, so students see that 20 means two tens and the answer is 56 rather than 38.
Units that teach this standard
Find where 1.NBT.C.4 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 Adding two-digit numbers within 100 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.