A Better Way To Help Kids Learn How To Multiply By 9

Multiplication-Division | 6 comments

My daughter is in the third grade and the other night she brought home the Finger Trick for learning “multiplying by 9.” It was not her teacher who taught her this, one of the students brought it into the classroom showing it to everybody and the teacher tried to help everybody understand why it worked. When I was in the class a few days afterwards, I noticed that there were a handful, maybe two handfuls, of students who were really reliant on using that Finger Trick.

In today’s vlog I want to talk about a better way to help kids learn how to multiply by 9 in our quest to build our math minds, so we can build the math minds of our students.

Watch the video below or read the transcript.

Join Build Math Minds PD Site for access to this training "Build Your Students' Multiplication Fact Fluency in 5 Days Using Number Talks"

Let’s Eliminate Tricks That Aren’t Real Strategies

I’m not going to spend time explaining the trick, because I think it’s so horrible, and because it’s a trick, not a strategy. So, let me take a few moments to talk about why. There is a simple difference between a trick that you’re teaching kids and a strategy that kids are developing. You can tell it’s a trick if at some point it stops working.

The finger trick for times 9 only works up to when you are multiplying by 10. Anything past that can not be done with the finger trick and it falls apart. It will last them for a short period of time, but if they don’t have an understanding of what times 9 is, it’s not going to help them moving forward.

This holds true when we are trying to teach the pattern associated with multiplying by nine.  There is a cool pattern, but it’s a trick. Teachers will try to help kids notice that the tens column is always one less than what you’re multiplying by.  So, if I’m multiplying the 9 by 2, my tens column is a 1. Or, when I’m multiplying 9 x 3, the tens column is 1 less than that, it’s a 2. And when I’m multiplying by 4, the tens column is a 3. Some people might wonder how is that helpful? Well, the other thing that kids have to know associated with this, is that the digits will add up to 9. So, if I know the tens column is a 1, what goes with 1 to make 9? Well, it’s an 8. The 2 is in the tens column, so what goes with 2 to make 9?

So they’re using that, but it’s a trick because it falls apart. Think about 12 x 9. If I’m following that pattern, the tens column should be 1 less. It should be 11, but it’s not. So, there’s actually a new pattern that develops, but then you’ve got to teach them a new pattern to help make sense of it.

 

What To Do Instead

I really want to encourage you instead of tricks, to look at a strategy that we can help develop for students that will last them forever. If we take the time to help kids see a relationship of multiplying by 9 and how that relates to multiplying by 10, it lasts forever. It’s such a cool strategy and has opened up my eyes to a world of appreciation for multiplying by nine.

A lot of kids will learn their times 10 fairly quickly. So you can do number talks like this, where you show 10 x 7, or as I like to say, 10 groups of 7. We know that that gives us 70. Then we’ll work to figure out what 9 groups of 7 gives us. Hopefully, we’re starting to get kids who will notice that it’s always just 1 group less of whatever we’re multiplying by.

So, if I have 10 groups of 4, I know that’s 40. What will 9 groups of 4 be? Well, we know it’s 36, but in kids’ eyes we want them to see it as 1 group of 4 less. As they progress forward, does it still work? If I’m doing 9 x 15, I could instead think 10 x 15 is 150, so I just need 1 group of 15 less than that, right?

And it’s not just for your multiplying by nines, it’s by multiplying with anything that ends with a 9. So, as I move forward and let’s say I’m doing 39 x 4. If I think about the next group of 10, what’s the next 10 that’s nearest? If I think about it as 40 groups of 4 that one’s 160, that’s easy for me. So, 39 groups of 4 should just be one group of 4 less.

That is a strategy that will work forever for our kids. It’s a wonderful strategy because it’s not a trick and It will work forever. So, I hope that this has given you a different way to approach multiplying by 9 with your students so that we can build their math minds instead of having them rely on tricks.

Want to spend 5 days learning how to develop a strategy for x9 with your students?  Join me for a 5 day training to Build Your Students’ Multiplication Fact Fluency in 5 Days using Number Talks.  Each day will be an online video training that shows how to use number talks with your students. Join Build Math Minds PD Site for access to this course.

Join Build Math Minds PD Site for HERE