1st Grade · Operations & Algebraic Thinking
1.OA.D.8: Determine the unknown whole number in an addition or subtraction equation
1.OA.D.8 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
Determine the unknown whole number in an addition or subtraction equation relating three whole numbers. For example, determine the unknown number that makes the equation true in each of the equations 8 + ? = 11, 5 = ? - 3, 6 + 6 = ?.
In plain words
Determine the unknown whole number in an addition or subtraction equation.
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 makes this equation true: 6 + ? = 9
- 2
- 3
- 4
Where students can get stuck
- Adding the two numbers they see wherever the box is, so 8 + ? = 11 gets 19 instead of 3.
- Subtracting 3 from 5 to get 2 for ? - 3 = 5, when the missing number is really 8.
- Freezing when an equation starts with the single number, like 5 = ? - 3, because it looks unfamiliar.
Effective teaching strategies
- Read 8 + ? = 11 as “What goes with 8 to make 11?” and have students build 8 with cubes and add more until they reach 11.
- Tell the story for ? − 3 = 5, with some cubes, then 3 taken away, and 5 left, and have students build the 5 and the 3 together to find the starting amount of 8.
- Write the same equation both ways, such as 5 = ? − 3 and ? − 3 = 5, so students see that the equal sign works from either side.
Units that teach this standard
Find where 1.OA.D.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.
enVision Mathematics
Go Math!
i-Ready Classroom Mathematics
Illustrative Mathematics
Into Math
Math Expressions
Don’t see your curriculum? Contact Bobby and we’ll see if we can help!
Share it with families
Families can read Missing numbers, like 8 + ? = 11 for a plain-language look at this skill, with 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.