Add or subtract 10 or 100
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.
In this video, your child will learn to add or subtract ten or one hundred.
The lesson stops at each question for your child's answer.
Three problems, each a little harder
Each problem is worked step by step. Your child will solve the last one before Bobby shows how.
Example 1
243 − 100 = ?
Example 2
795 + 10 = ?
Your child's turn
580 − 10 = ?
The three key steps
- 1Add or take 10: tens change
- 2Add or take 100: hundreds change
- 3Ten tens? Make a hundred.
Mistakes to listen for
- 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.
The lesson shows every step, so your child sees why it works. Read our parent's guide to adding or taking away 10 or 100 mentally →
Is your child ready to practice what they've learned?
Bobby's Math Club offers unlimited practice for free.
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More 2nd grade Numbers & Place Value, with questions your child answers along the way.
Questions parents ask
Does my child need an account to watch?
No. You can watch this on any device at any time. If your child would like to practice adding or taking away 10 or 100 mentally, and you'd like parent-friendly reports on your child's progress and growth, you can join Bobby's Math Club for unlimited free practice.
What happens when my child answers a question?
A right answer gets a cheer, and the lesson carries on. If your child isn't sure, they can tap "I don't know yet", and Bobby gives the answer and shows another example. Nothing is scored or stored.
Is this the same lesson that's on YouTube?
Yes. Here, the lesson stops at each question and waits for your child to answer. On YouTube, each question stays on screen for a few seconds, and then the lesson plays on.
Where can I learn more?
There's a short, kid-friendly lesson like this one for every Pre-K to 5th grade math skill, all in our video library. Our parent's guide to adding or taking away 10 or 100 mentally has more on where kids get stuck and how you can help at home. In the club, your child practices at just the right level, about five minutes a day, and you get parent-friendly reports as they grow. Learn more about how it works →
Practice adding or taking away 10 or 100 mentally
Questions at your child's level, about five minutes a day, on adding or taking away 10 or 100 mentally and whatever PK-5 skills they need to develop. Unlimited practice, free forever.
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