Multiplying by tens, like 4 x 70
In 3rd grade, children learn to multiply one-digit numbers by multiples of ten, such as 4 x 70 = 280, by thinking of it as 4 groups of 7 tens.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 3rd 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
3 × 90 is the same as 3 groups of 9 tens. How many tens is that in all?
- 12 tens
- 27 tens
- 270 tens
Why this matters
This is the bridge your child crosses between knowing times facts and multiplying bigger numbers. In 4th grade, your child will find 4 x 73 by adding 4 x 70 and 4 x 3, and work out 30 x 40 with the same reasoning. Understanding tens, rather than a rule about adding a zero, also keeps things working when decimals arrive in 5th grade.
Where kids can get stuck
- Multiplying the digits and forgetting the tens, writing 4 x 70 = 28 instead of 280.
- Writing the wrong number of zeros when the fact already ends in zero, so 5 x 80 becomes 40 or 4,000 instead of 400.
- Using the add-a-zero trick without knowing why, so they cannot tell whether 3 x 60 = 180 is reasonable.
How you can help
- Count out dimes and ask your child how much 6 dimes worth 10 cents each are worth, then move to bills like 3 tens worth $10 each for $30.
- Ask how many minutes are in a few hours if you think of an hour as 60 minutes, such as 3 x 60 = 180 minutes, while planning a trip.
- Use packs of 10 pencils or crayons to show that 4 packs of 20 is the same as 4 x 2 tens, which is 8 tens, or 80.
If it’s helpful, you can sign up for free, adaptive practice in Bobby's Math Club.
Questions parents ask
Why doesn't the school teach my child to add a zero?
Adding a zero gives the right answer with whole numbers, but it hides the reason and fails later. In 5th grade, 4 x 0.7 is 2.8, not 0.70. Thinking of 70 as 7 tens helps your child understand why the answer to 4 x 70 is 28 tens, and that reasoning keeps working as numbers change.
What does multiples of 10 mean?
A multiple of 10 is a number you get by counting by tens: 10, 20, 30, and so on up to 90 in 3rd grade. Each one is a number of tens, so 50 is 5 tens and 90 is 9 tens. Your child multiplies these by one-digit numbers like 3 or 8, using times facts they already know.
How does this help with multiplying bigger numbers later?
In 4th grade, your child will multiply numbers like 6 x 47 by splitting 47 into 40 and 7. They will need 6 x 40 = 240 and 6 x 7 = 42, then add to get 282. Being comfortable multiplying by tens now makes that next step much smoother.
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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 3.NBT.A.3: “Multiply one-digit whole numbers by multiples of 10 in the range 10-90 (e.g., 9 x 80, 5 x 60) using strategies based on place value and properties of operations.” Learn how Bobby's Math Club can help your students grow. (Common Core 3.NBT.3)