Subtracting within 20
In 1st grade, children learn to subtract within 10 quickly and accurately; to find differences, the answers to subtraction problems, within 20 by stopping at ten; and to use addition facts they know.
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
What is 10 - 6?
- 4
- 16
- 5
Why this matters
Subtracting gives your child a way to answer the questions they ask all day: how many are left, and how many more until it's their turn? Facts like 13 - 4 lead straight into subtracting within 100 next year. And once your child sees that 12 - 8 undoes 8 + 4, every addition fact they know becomes a subtraction fact too. You'll notice it when cookies get eaten or toys are shared.
If subtracting within 20 still takes a lot of counting, your child can review adding within 20, since every subtraction fact has an addition fact behind it.
Where kids can get stuck
- Saying the starting number as the first count back, so 13 - 4 goes "13, 12, 11, 10" and comes out as 10, not 9.
- Hearing "how many more" as a signal to add, so "How many more is 15 than 9?" gets the answer 24 instead of 6.
- Counting up to find 12 - 8 but answering with the number they land on, 12, rather than how many they counted, 4.
How you can help
- Line up 14 toy cars and send 5 to the garage, 4 first and then 1 more. Ask your child how many are left after the 4 go, and again after the last one.
- Draw a chalk number line from 0 to 20 on the sidewalk. Your child stands on 12 and hops back 5, noticing that 2 hops bring them to 10 and 3 more land on 7.
- Ask how many more during a board game. If your child is on space 9 and needs to reach 16, have them count up from 9 to find the 7 spaces to go.
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 to subtract by going through ten?
Ten is a friendly stopping place. For 13 - 4, your child takes away 3 to reach 10, then 1 more to reach 9. Each step is small and easy to hold in mind. The same idea works later for 53 - 7 and beyond. It also builds the habit of breaking a number into handy parts, which is the heart of doing math in your head.
Why is my child's subtraction answer always one off?
This usually comes from counting back. For 15 - 3, your child says "15, 14, 13" and answers 13, because they counted the starting number. The fix is to count the steps, not the numbers: hold up a finger for each jump back, saying "14, 13, 12" as three fingers go up. Seeing three fingers shows them where to stop.
Should my child count back or count up to subtract?
Both work, and the best choice depends on the numbers. Counting back is quick for 13 - 2, when only a little is taken away. Counting up is quicker for 13 - 9, since 9 to 13 is only four steps. Encourage your child to look at the numbers first and pick the shorter path. Over time, many of these facts become known without counting at all.
Make learning math fun
Bobby's Math Club turns 1st grade math practice into something fun to do together.
- Unlimited free practice
- Practice that starts where your child is and grows with them
- 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.
Ready to watch your child’s confidence grow?
Your 1st grade child can start today with free, unlimited practice that grows with them.
Join the clubNote for teachers: This skill is covered by Common Core math standard 1.OA.C.6: “Add and subtract within 20, demonstrating fluency for addition and subtraction within 10. Use strategies such as counting on; making ten (e.g., 8 + 6 = 8 + 2 + 4 = 10 + 4 = 14); decomposing a number leading to a ten (e.g., 13 - 4 = 13 - 3 - 1 = 10 - 1 = 9); using the relationship between addition and subtraction (e.g., knowing that 8 + 4 = 12, one knows 12 - 8 = 4); and creating equivalent but easier or known sums (e.g., adding 6 + 7 by creating the known equivalent 6 + 6 + 1 = 12 + 1 = 13).” Learn how Bobby's Math Club can help your students grow. (Common Core 1.OA.6)