10 more or 10 less in your head
In 1st grade, children learn to find 10 more or 10 less than a two-digit number in their heads and to explain why only the tens digit changes.
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 number is 10 more than 33?
- 43
- 34
- 23
Why this matters
When your child can say right away that 10 more than 43 is 53, it shows they are thinking in tens, not ones, and adding and subtracting get much faster. This mental skill is the start of jumps like 47 + 30 in 2nd grade, where children also find 100 more or 100 less than a number. It makes everyday mental math easier too, like counting money in dimes.
Where kids can get stuck
- Counting on by ones, ten separate times, to find 10 more than 56, and often miscounting along the way.
- Changing the wrong digit, saying 10 more than 34 is 35, because adding seems to change the last digit.
- Getting stuck near the ends, like 10 less than 15 or 10 more than 92, where the answer has a different number of digits.
How you can help
- Play a quick car game: you name a number, like 67, and your child says 10 more and 10 less. Ask them to explain how they know without counting.
- Use dimes and pennies. Show 38 cents with 3 dimes and 8 pennies, then add or remove one dime and ask how much money there is now.
- Count by tens starting from a number that does not end in zero, such as 4, 14, 24, 34, while climbing stairs or jumping rope, so the pattern becomes familiar.
If it’s helpful, you can watch the kid-friendly video below, or sign up for free, adaptive practice in Bobby's Math Club.
Kid-friendly explanation
Questions parents ask
Why is my child learning to find 10 more and 10 less mentally?
Finding 10 more or 10 less without counting shows that your child understands a number is made of tens and ones. It is also a very useful mental math skill. Adding 10 becomes a quick change to the tens digit instead of a long count, which makes larger addition and subtraction much easier.
What does explain the reasoning mean for a 1st grader?
It means your child can say why the answer works, in simple words. For example: "10 more than 46 is 56, because 46 has 4 tens and I added 1 more ten, so now there are 5 tens and still 6 ones." A short explanation like this shows understanding, not memorized patterns.
What comes next after 10 more and 10 less?
In 1st grade, children also subtract multiples of 10, like 70 - 30. In 2nd grade, they find 10 more, 10 less, 100 more, and 100 less than numbers between 100 and 900, such as 100 more than 345 is 445. The same idea of changing one digit carries through.
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
- 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 1.NBT.C.5: “Given a two-digit number, mentally find 10 more or 10 less than the number, without having to count; explain the reasoning used.” Learn how Bobby's Math Club can help your students grow. (Common Core 1.NBT.5)