Subtracting fluently within 20
In 2nd grade, children learn to subtract within 20 in their heads, quickly and accurately, and to use an addition fact they know, like 8 + 6 = 14, to find 14 - 8.
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
Which subtraction has the same answer as 10 - 3?
- 15 - 8
- 13 - 3
- 15 - 7
Why this matters
If subtraction facts come easily, your child can focus on what a problem is asking instead of counting backward. Facts like 15 - 7 sit inside bigger problems this year, such as 65 - 27. They're also how your child works out how many are left in a pack, how many points a team still needs, or how many days remain until the weekend. Each fact they know saves time on every bigger problem that uses it.
If subtraction facts come slowly, reviewing adding fluently within 20 can help, since each sum your child knows can be run backward.
Where kids can get stuck
- 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.
How you can help
- Use snack time: "There were 13 grapes and you ate some. Now there are 5. How many did you eat?" Then ask your child how they knew it was 8.
- Make a countdown on the calendar to something your child is looking forward to, like a visit 18 days away. Every few days, ask how many days have gone by and how many are left.
- Play a card game with cards from ace to 9. Say a number from 11 to 18, flip a card, and race to name what's missing: for 16 and a 7, the answer is 9.
The kid-friendly video below is a nice one to watch together. For more, Bobby's Math Club has free practice at your child’s level.
Kid-friendly explanation
Questions parents ask
Why is my child learning to subtract by thinking of addition?
Every subtraction fact is an addition fact turned around. If your child knows 6 + 8 = 14, they also know 14 - 8 = 6 and 14 - 6 = 8. Asking "8 and what make 14?" is often faster and more reliable than counting back. It also means far fewer separate facts to carry. So the sums your child knows by heart do double duty.
Does my child need to memorize subtraction facts too?
By the end of 2nd grade, children are expected to know their addition facts from memory. Subtraction facts are expected to be quick and accurate, but many children work them out with a quick strategy rather than recalling each one on its own. That's fine. Over time, many subtraction facts start to feel known as well, especially the ones your child uses most.
Why are subtraction facts harder for my child than addition facts?
Many children find them slower at first. Adding moves forward, the way children first learned to count, while subtracting asks them to work backward or find a missing part. Start with facts that stay above ten, like 17 - 4, then move to ones that cross ten, like 13 - 5, using 10 as a stopping point: 13 - 3 is 10, then 2 more is 8.
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
- Level-ups, badges, and a cheer for every win
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.OA.B.2: “Fluently add and subtract within 20 using mental strategies. By end of Grade 2, know from memory all sums of two one-digit numbers.” Learn how Bobby's Math Club can help your students grow. (Common Core 2.OA.2)