2nd Grade · Operations & Algebraic Thinking
2.OA.B.2: Fluently add and subtract within 20
2.OA.B.2 is a 2nd 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
Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers.
In plain words
Fluently 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
What is 9 + 6?
- 14
- 15
- 16
Where students can get stuck
Fluently add within 20.
- Counting on fingers or by ones for facts like 8 + 6, which slows every larger problem and invites slips.
- Confusing near neighbors, such as answering 7 + 8 = 16 because the double 8 + 8 = 16 feels close.
- Treating 4 + 9 and 9 + 4 as two separate facts to learn, not seeing that swapping them keeps the sum at 13.
Fluently subtract within 20.
- Counting back by ones for 15 - 7, which is slow and easy to lose track of, so the answer comes out 7 or 9.
- Racing through facts like 17 - 3 but slowing to a crawl on ones that cross ten, like 13 - 7 or 12 - 5.
- Knowing 9 + 5 = 14 by heart without noticing that the same fact answers 14 - 9 and 14 - 5.
Effective teaching strategies
- Use a double such as 6 + 6 = 12 to find 6 + 7 by adding one more, so students build new facts from ones they already know.
- Show 8 + 6 on two drawn ten frames and move 2 to fill the first frame, so students see 10 + 4 = 14 and learn to make a ten.
- Ask “what adds to 8 to make 14?” to turn 14 - 8 into a missing-addend question, and have students check the answer by adding it back.
Units that teach this standard
Find where 2.OA.B.2 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
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
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.