Adding or taking away 10 or 100 mentally
In 2nd grade, children learn to add or subtract 10 or 100 in their heads from numbers between 100 and 900, seeing which 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 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
What is 10 less than 100?
- 110
- 90
- 10
Why this matters
Adding 10 or 100 in their head shows that your child truly understands how numbers are built. Knowing which digit changes, and why, makes mental math much faster, like jumping from 456 to 556 to 656 instead of counting. In 3rd grade the same understanding is the base for rounding to the nearest 10 or 100, which is how children learn to estimate and check their answers.
Where kids can get stuck
- Changing the wrong digit, such as saying 100 more than 432 is 442.
- Getting stuck when the tens digit is 9, answering 2103 or 203 for 10 more than 293 instead of 303.
- Counting on by tens to find 100 more instead of changing the hundreds digit, and losing count along the way.
How you can help
- Play a quick game in the car: you say a number like 517, and your child says 10 more, 10 less, 100 more, and 100 less, explaining which digit changed.
- Use coins and bills: show 346 cents as 3 dollars, 4 dimes, and 6 pennies, then add a dime or a dollar and ask your child how many cents there are now.
- Look at house numbers on a walk and ask what number would be 10 or 100 more, especially when the tens digit is a 9, like 395.
Want a little more? Watch the kid-friendly video below together, or try Bobby's Math Club for free practice that adapts to your child.
Kid-friendly explanation
Questions parents ask
Why is my child learning to add 10 or 100 in their head?
Adding or subtracting 10 or 100 mentally shows that your child understands what each digit in a number is worth. It is also a useful shortcut for larger addition and subtraction. If your child knows that 100 more than 562 is 662 without counting, they are thinking in hundreds, tens, and ones.
Why is 10 more than 395 so hard?
Adding 10 to 395 means the tens go from 9 to 10, which makes a new hundred. So the answer is 405, and two digits change. Children often write 3105 or 305. Showing it with play money, where 10 dimes are traded for a dollar, makes the jump easier to understand.
How is this different from what my child did in 1st grade?
In 1st grade, children found 10 more or 10 less than two-digit numbers, such as 10 more than 46 is 56. In 2nd grade, they work with three-digit numbers between 100 and 900 and add 100 more and 100 less too. The idea is the same, extended to a new place.
Make learning math fun
Bobby's Math Club turns 2nd grade math practice into something fun to do together.
- Unlimited practice, completely free
- Questions matched to your child’s level, a few minutes a day
- 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.8: “Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900.” Learn how Bobby's Math Club can help your students grow. (Common Core 2.NBT.8)