Subtracting by finding the missing part
In 1st grade, children learn to think of subtraction as finding a missing part, so 10 - 8 becomes the question "what do I add to 8 to make 10?"
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 1st grade practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
If 5 + 3 = 8, what is 8 - 3?
- 3
- 5
- 8
Why this matters
This idea can make subtraction feel far less daunting for your child, because every addition fact they know becomes a subtraction fact too: 8 + 2 = 10 tells them that 10 - 8 = 2. Counting up is also how people make change with money. In 2nd grade, the same thinking makes problems like 53 - 48 quick: count up 2 to reach 50, then 3 more.
Where kids can get stuck
- Counting backward for every problem, even 15 - 13, and losing track, landing on 3 or 1 instead of 2.
- Counting the starting number as a step, saying "8, 9, 10, that's 3" for 10 - 8 instead of 2.
- Treating 10 - 4 as a brand-new problem, without noticing that the well-known fact 6 + 4 = 10 already answers it.
How you can help
- Fill an egg carton with 8 eggs or small toys and ask how many more are needed to fill 10 spaces. Then say the matching subtraction aloud together: 10 take away 8 is 2.
- Play a missing-part game with 9 coins. Hide some under your hand, show the rest, and ask how many are hiding. If 6 show, your child works out that 3 are hidden.
- Count up while cooking or baking: "The recipe needs 7 spoons of flour and we have put in 5. How many more spoons?" Let your child count up from 5 to 7.
The kid-friendly video below explains it in a few minutes, and Bobby's Math Club offers free practice that grows with your child.
Kid-friendly explanation
Questions parents ask
Why is my child learning subtraction by adding?
Thinking "8 plus what makes 10?" uses facts your child already knows to solve subtraction. It is often faster and more accurate than counting back, especially when numbers are close. It also builds understanding that addition and subtraction are linked, which supports mental math and problem solving in every later grade.
Should my child count back or count up to subtract?
Both are useful, and children benefit from knowing when each is easier. Counting back works well for taking away a small amount, like 12 - 2. Counting up works better when the numbers are close, like 12 - 9. Asking your child which way feels quicker for a given problem helps them make sensible choices.
How will I know my child understands this?
A good sign is when your child solves 10 - 7 by saying something like "7 and 3 make 10, so it's 3." Another is being able to write the matching addition for a subtraction problem. If they still count back every time, that is normal early in the year, and practice with real objects helps the link form.
Make learning math fun
Bobby's Math Club turns 1st grade math practice into something fun to do together.
- Unlimited free practice
- Questions matched to your child’s level, a few minutes a day
- Hints when they are stuck, and a celebration when it clicks
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.
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Join the clubNote for teachers: This skill is covered by Common Core math standard 1.OA.B.4: “Understand subtraction as an unknown-addend problem. For example, subtract 10 - 8 by finding the number that makes 10 when added to 8.” Learn how Bobby's Math Club can help your students grow. (Common Core 1.OA.4)