1st Grade · Operations & Algebraic Thinking
1.OA.C.6: Add and subtract within 20
1.OA.C.6 is a 1st grade Common Core math standard in Operations & Algebraic Thinking. 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 and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 - 4 = 13 - 3 - 1 = 10 - 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 - 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).
In plain words
Add and subtract within 20.
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 6
Mia has 7 toy cars. Her friend gives her 2 more toy cars. How many toy cars does Mia have now?
- 8
- 10
- 9
Where students can get stuck
Add within 20.
- Counting by ones for every sum, which works for 3 + 2 but gets slow and error-prone for 9 + 7.
- Splitting a number wrongly when making ten, turning 8 + 5 into 8 + 2 + 2 and getting 12 instead of 13.
- Adjusting a double the wrong way, so 7 + 8, thought of as 8 + 8 = 16, comes out as 17 instead of 15.
Subtract within 20.
- Saying the starting number as the first count back, so 13 - 4 goes "13, 12, 11, 10" and comes out as 10, not 9.
- Hearing "how many more" as a signal to add, so "How many more is 15 than 9?" gets the answer 24 instead of 6.
- Counting up to find 12 - 8 but answering with the number they land on, 12, rather than how many they counted, 4.
Effective teaching strategies
- Model 8 + 5 by filling a drawn ten frame, moving 2 of the 5 over to make 10, and writing 8 + 2 + 3 = 10 + 3 = 13.
- Hold up a known double such as 7 + 7 = 14 and ask students to use it to solve 7 + 8 by adding one more.
- Pair 8 + 4 = 12 with 12 − 8 = 4 and ask students to use the sum they know to solve the subtraction without counting back.
Units that teach this standard
Find where 1.OA.C.6 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
Go Math!
i-Ready Classroom Mathematics
Illustrative Mathematics
Math Expressions
Reveal Math
Don’t see your curriculum? Contact Bobby and we’ll see if we can help!
Share it with families
Each skill has its own guide for families, with a plain-language look at the skill, three questions to try together, and ideas for helping at home.
Help every student practice problem solving 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.