# Ep 23: The Commutative Property

November 24, 2020 Pam Harris Episode 23
Math is Figure-Out-Able with Pam Harris
Ep 23: The Commutative Property
Chapters
Math is Figure-Out-Able with Pam Harris
Ep 23: The Commutative Property
Nov 24, 2020 Episode 23
Pam Harris

How can we help students feel comfortable with the commutative property? In this episode Pam and Kim go through some addition and multiplication problems to demonstrate how they think and reason using the the commutative property.
Talking Points

• The commutative property in counting
• The commutative property in addition
• Using the commutative property for percentages

Find the transcript HERE

How can we help students feel comfortable with the commutative property? In this episode Pam and Kim go through some addition and multiplication problems to demonstrate how they think and reason using the the commutative property.
Talking Points

• The commutative property in counting
• The commutative property in addition
• Using the commutative property for percentages

Find the transcript HERE

Pam Harris:

Hey fellow mathematicians. Welcome to the podcast where math is figure-out-able. I'm Pam.

Kim Montague:

And I'm Kim.

Pam Harris:

And we're here to suggest that mathematizing is not about mimicking, or rote memorizing. It's about reasoning; about creating and using mental relationships. We aree all about empowering teachers, students, parents, and everybody, to empower them to think and reason mathematically. We answer the question, if not algorithms, then what? Alright, in today's episode, we're going to highlight the commutative property, and how it can make some mental computation really cool.

Kim Montague:

Mhmm.

Pam Harris:

Okay, so when kids start to solve problems using counting strategies, because that's where they first do this, for solving problems, these counting strategies get really literal. And they always start with the first number, they sort of act out the problem. And so that means they're going to act it out in order, because that's what makes sense to them.

Kim Montague:

Yeah. So for example, if you had a problem, like Javan had four pencils, she found eight pencils on the floor and picked them up, how many pencils does she have now? Kids are going to often solve that by starting with those four pencils in the problem. And then we're really, really happy when students realize that they can count on from the larger number starting with the eight, because then they have less to count on.

Pam Harris:

Yeah, sure enough, so instead of a student saying, okay, four pencils. And now I'm going to pick these pencils up: 5, 6, 7. And sort of keeping track of those extra eight, a student can just start with the 8. 8, 9, 10, 11, 12. They have to count on less, they have to count on four, instead of counting on all of the eight. So counting on makes sense. We kind of want to bring out that with students, Kim, what are some ways that teachers can encourage young students to construct this idea, help them construct the idea of counting on from the larger?

Kim Montague:

Well, first of all, they just need to know that it's a thing, right? So that's one reason why we share strategies. So I feel like we'd solve a problem, and I'd have one kid share who did all that counting on starting with the smaller number. And then I'm going to invite someone to share, who saw that they could just start with the eight and just count on four more. And I would celebrate hard how sophisticated their thinking was, and ask the students to consider which one they'd like to do next time, right. So there's also some times where you may need to be even more explicit. I was thinking about a few years ago, when I worked with a student who was struggling to move from counting on to using relationships. And we were doing some individual work. And I found it really interesting, when I was kind of poking around a little bit to see where some of those gaps were, one of the things that I realized was that he wasn't ever counting on from the bigger number. And the reason that he wasn't doing that was because he didn't yet trust that problems, like four plus eight and eight plus four, would give him the same result, he didn't see that they were equivalent. For him, and maybe for some of your students, it's helpful to see that equivalence of problems like eight plus four and four plus eight, so I would absolutely have my rekenrek or my number rack out and ask, what does a four plus eight look like. And then for an eight plus four, it's just that same number rack flipped over, right? So if I have for on top and eight on the bottom to represent four plus eight, then if I flip the rack over, that's going to represent eight plus four, eight on top and four on the bottom. And those are equivalent, they can actually see no beads have been added or removed. They're just arranged differently on top and bottom. That's a really nice sort of visual concrete way for students to go.

Pam Harris:

Kim Montague:

Yeah, as long as we're open to the idea that I would rather solve that 24 plus 99 as 24 plus 100. And then backup one because I really do like the over strategy.

Pam Harris:

I will never forget, when you told me that one day, I was like, so excited about being able to give and take and this idea of 99, and you're like, Well, actually, I like to over those, which is totally reasonable. Right? Totally cool. You can completely do that. And it's a great relationship. But yeah, it's not my go to. But yeah, we will, we will allow that. I had a feeling you would say that. Okay. So, Kim, what's another place where using a community property might come in handy.

Kim Montague:

So if our listeners were available last week, they learned about the swapping strategy for addition, and that's based on the commutative property of addition as well.

Pam Harris:

Right. So in Episode 22, we talked about the swapping strategy, and in case people didn't catch episode 22, although you really ought to go listen to it, it's pretty great. Kim will you remind us about the swapping strategy a little bit? Yeah, sure. So that's like when you're given a problem, like 59 and 92. And you want to just read rearrange the place values to create an equivalent problem. So like 59, and 92, could be rearranged to create a problem of 99 and 52. And then you can sort of do either one of the two strategies that you and I just talked about, right? Because I have 99 plus anything and so 99 plus 42 is delightful to solve. Cool. All right, what's another community property gem?

Kim Montague:

Oh, probably one of my favorites. So 10 times anything can be thought about as that many 10s. So like, go ahead.

Pam Harris:

Kim Montague:

So Pam, what if I asked you to find 23 times 99?

Pam Harris:

So find 23 99s? Yuck. Okay, cool. So this is a little complicated hold on your hat, because I'm actually going to use the commutative property twice here. So the first thing when I think about finding 23 99s, is that I don't really want to find 23 99s, I actually want to find 9920 threes, because I think that's going to be easier for me because 99 is so close to 100. If I can find 100 23s then 99 23s will just be one less 23. Okay, so I'm going to focus on finding 100 23s in order to find 99 23s. So that's the first time that I use the commutative property. But now just like we just said, in order to find 100 23s, I don't actually want to think about 100 23s, I want to think about 23 100s, and 23 100s is just the number 2300. Right? It's like 23 times 100 is twentythre-hundred or twothousand-threehundred. So I've sort of used it twice. So now that I have 100 23s. I can think about that as 23 100s. Wait, Nope, sorry, I can think about 23 100s as 100 23s . And so then I can find 99 of them by just getting rid of one 23. That might be a little confusing, but I sort of use the community property twice in there in order to find 99 23s rather than 23 99s.

Kim Montague:

That was really well said and it might give our listeners a little bit of insight because you're so able to verbalize that a little bit of insight into why the commutative property is kind of complex and not just, hey, do this thing kids.

Pam Harris:

Yeah, especially if we just decided, alright, I'm going to tell you what the community property is. And I'm going to write up an example on the board. And I want you to memorize the name of it. So we are not suggesting that, that doesn't work. It doesn't give kids the flexibility that you just heard us using and all these examples, what we really need to do is engage kids in the problem solving process, and share those strategies and use problem strings to build the communitive property and use concrete examples like flipping the number rack or with multiplication, we really want to be able to represent multiplication on rectangles, because then we can rotate those rectangles and we can notice that if I have a 23 by 99, that's going to have the same area as a 99 by 23. So some of those visual models are gonna be really important in helping kind of concretely build that communitive property in students so that we can use it in examples like we just used. Alright, so I've got a finale for us today, Kim.

Kim Montague:

Okay.

Pam Harris:

All right. So in episodes, 20, and 21, we talked all about percentages, we played with percentages, and we threw out lots of percent problems, we actually had an ulterior motive in there. An ulterior motive where we kind of had a goal because you know, there's always a BAM in there somewhere, there's always something that we're kind of leading towards something that we want to be able to refer back to, or something that we want to be able to kind of keep listeners engaged, keep them, keep them on the edge of your seats, try to figure out what our next BAM is going to be.

Kim Montague:

So yeah, let me list a couple of those problems and answers and see if maybe you notice a pattern, maybe one that has something to do with the commutative property. So we asked 12% of 50. 50% of 12. And those were both six. And we also asked 45% of 10. and 10% of 45. And those were both 4.5.

Pam Harris:

So we have some equivalent answers. What's going on? All right, so in percents, there's a multiplicative relationship between the whole and the percent. So is 10% of 45 really equivalent to 45% of 10? Yes, because 10 times 45 is equivalent to 45 times 10. Wow, that is so cool.

Kim Montague:

It is so cool. So here's a really cranky problem, would this be one that you would want to use the commutative property? And that would make it really nice. What about 67% of 20?

Pam Harris:

Okay, so since I know it's a thing, since I know, I can choose to do 67, I can find 67% of 20. Or I can choose to do 20%, of 67. Since I know that's a thing, since I have that choice, because I understand the communitive property of multiplication, I'm going to choose to do 20% of 67 every time! Because I can find 10% of 67, that's just six and seven tenths or 6.7. Then I can double that because that's 10%. So double that to get 20%. So double 6.7, that's just 13.4, double 6.7 is 13.4. And so 20% of 67 is 13 and four tenths or 13.4. In other words, I can now look at a percent problem, find some percent of a number and I can ask myself, do I want to find that? The thing that's given to me like 8% of B? Or do I want to use the community property to find B percent of A? So cool. So let's try another one.

Kim Montague:

Okay.

Pam Harris:

All right. So I'm going to try to find a problem that it won't really help to use the commutative property, and we'll see how that goes. So Kim, how about 82% of 19?

Kim Montague:

Oh, sounds awful. 82% of 19. So I know that I can do 82% of 19 or 19% of 82. Those are my choices. Ah, and at first glance, they don't seem really nice. But you know what, I would rather swap here, because I would rather solve 19% of 82. Because I can find 20% of 82 and then get rid of the 1%.

Pam Harris:

Oh, nice. So instead of finding 82% of it, you're going to find 19% of 82 by finding 20% and getting rid of 1%. Okay, cool. So I tried to come up with a problem where you wouldn't want to use the community property, but you still did. Let me see if I can make it even harder. So we started with 82% of 19. What about 82% of 17?

Kim Montague:

So 82% of 17 or 17% of 82. You know what? 82% is not bad because I can just do 2%, 4%, 8%, 80%. Like double double double times 10. So 2% of 17, 4% of 17. And I'm not going to actually throw out the numbers right here, but the strategy that I would use is double, double, double, times ten to get the 80%. And then just tack on the 2%. To find 2% of 17.

Pam Harris:

Nice. Yes, yes. Totally cool. Well, okay, let me give you another one. What about 83%? of 17?

Kim Montague:

I'm Not quite as nice... 83% of 17 or 17%. of 83, I think you've got one. I think that's, that's a nice problem. But we can just chunk some pieces just like we've done before, right?

Pam Harris:

Sure. It's just maybe neither of them are pulling at you to use the commutative property either way?

Kim Montague:

Nah, either way.

Pam Harris:

Cool. Okay, cool.

Kim Montague:

So we've talked today about the commutative property and where kids can use it at different ages. It's not that they have to name the property, right. You said that earlier. We're not about naming the property, but that they can use the property and figure it with some relationships. cool.

Pam Harris:

The commutative property of addition and multiplication! Who knew? So helpful! We hope you enjoyed our journey today. We love it when you like the podcast and review it, that helps more people see it. Check us out on the website, mathisFigureOutAble.com. We'd love for you to join us on #MathStratChat on Wednesday evenings for the global number talk. So if you're interested to learn more math, and you want to help students become mathematicians where they mathematize their world, then the Math is Figure-Out-Able Podcast is for you. Because math is figure-out-able! out