Explaining why subtraction strategies work
In 2nd grade, children learn to explain why a subtraction strategy works, to show why a ten can become ten ones, and to say why adding back checks the answer.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 2nd 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
8 tens take away 3 tens is 5 tens. So what is 80 - 30?
- 50
- 5
- 77
Why this matters
Talking through subtraction helps your child trust their own thinking, especially on problems where a ten has to be broken apart. If they can say why 52 - 17 needs a trade, they're less likely to subtract the wrong way round. And the reasons carry forward: the same ideas explain subtracting bigger numbers later this year and next, and checking work with addition for years to come.
If giving reasons for subtraction feels hard, your child can start with explaining why addition strategies work, which uses the same tens and ones.
Where kids can get stuck
- Saying "you can't take 7 from 2" for 52 - 17, instead of explaining that a ten becomes ten ones to make 12.
- Taking 20 from 45 to find 45 - 19, then taking 1 more away instead of adding it back, and getting 24, not 26.
- Believing that order doesn't matter in subtraction, as it doesn't in adding, so 8 - 15 seems the same as 15 - 8.
How you can help
- Make a deliberate mistake, like saying 54 - 18 is 44, and ask your child to explain to you what went wrong and how to fix it.
- Draw 52 as 5 sticks and 2 dots on a napkin, then ask your child to take away 17 and explain, out loud, why one stick has to become 10 dots first.
- After your child finds how many more points you need in a game, like 60 - 37 = 23, ask them to show why adding 23 back to 37 proves it.
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 does my child explain subtraction with drawings of tens and ones?
A drawing makes the trade visible. If there aren't enough ones, your child can show one ten turning into ten ones and see exactly why the answer works. The quick marks of a written method, like crossing out a 5 and writing a 4, stand for that same trade. A child who has drawn it knows what those marks mean.
How do I know if my child really understands subtraction?
Ask them to explain a problem in their own words, then check it another way. If your child can say why 63 - 28 needs a ten broken into ones, and can show that 35 + 28 brings them back to 63, they understand it. Getting the right answer is a good sign. Being able to say why is a stronger one.
My child can explain adding but not subtracting. Is that common?
Yes, and it makes sense. Subtraction is harder to explain, because the trade goes the other way: a ten is broken apart rather than made, and the order of the numbers matters. Give it time, and use objects. Start with problems that need no trade, like 48 - 23, then move to ones that do, like 42 - 27, and ask what changed.
Make learning math fun
Bobby's Math Club turns 2nd grade math practice into something fun to do together.
- Unlimited practice, completely free
- A few minutes a day, at just the right level
- You see their progress and what to work on next
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 2.NBT.B.9: “Explain why addition and subtraction strategies work, using place value and the properties of operations.” Learn how Bobby's Math Club can help your students grow. (Common Core 2.NBT.9)