# Ep 64: Fun with Division

September 07, 2021 Pam Harris Episode 64
Math is Figure-Out-Able with Pam Harris
Ep 64: Fun with Division

There is so much to division! To show that, Pam and Kim go through a quick Problem String that builds a less known, but super powerful, division strategy.
Talking Points:

• The importance of hydration
• Using equivalent ratios to solve gnarly division problems

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 make the case that mathematizing is not about mimicking steps or rote memorizing facts. But it's about thinking and reasoning, about creating and using mental relationships. We take the strong stance that not only are algorithms not particularly helpful in teaching, but that mimicking algorithms actually keeps students from being the mathematicians they can be. We answer the question, if not algorithms and step by step procedures, then what?

Kim Montague:

So last week, we had a fantastic division challenge for the world, right? So much fun.

Pam Harris:

Oh, my gosh, that was fun.

Kim Montague:

So we decided that we're going to tackle a little bit of division today.

Pam Harris:

I mean, we are not done with division. Because let's be clear, there's a lot. Like division is interesting. And it's complicated. And it's complex. Lots of ins and outs, and therefore even more fun, right? Like we can really have -

Kim Montague:

And super Figure-Out-Able.

Pam Harris:

Super, super Figure-Out-Able, not rote-memorizable. Nicely said. Okay. So, you know, we're gonna be a little transparent here and say that sometimes when we do the podcast, we plan it out a little bit more. And I will actually tell Kim the problem that I'm about to give her so she can think through it a little bit, we've done that. We've also done it where we've literally thrown out the problem, and neither of us have done a lot of thinking - uh that's not true. I think I've always thought about it at least a little bit. Or if you've given me the problem, you've thought about it, but the other person hasn't heard it today. Today is the cold case. I've created some division stuff. And Kim has not heard these before. So this will be a little fun and exciting. And here we go. All right. Totally willing? Alright, hey, Kim, if we were going on a field trip, and we had 500 students, so it's kind a big, the whole school is going on the field trip. And we decided for the, or maybe it's field day, I don't know, give me a good context. It's Field Day. Because for whatever reason, we need water bottles, and we were taking stock of what we have. And we've got a bunch of water bottles. And we know that right now for those 500 students, we have 250 water bottles, and I'm curious how many water bottles each of those students can have?

Kim Montague:

Okay, cool. So I grabbed my pencil and paper, just in case, but 500 Kids 250 water bottles, we don't have enough water balls for everybody.

Pam Harris:

So how many water bottles is there per kid?

Kim Montague:

One half of a water bottle per kid.

Pam Harris:

Like that's not very good, right? We can only give them half of a water bottle per kid. So hmm. Okay. So it gets even worse Kim because we don't have enough. But what if I had 1000 students? And still we only had those 250 water bottles? How many water bottles would each kid get?

Kim Montague:

Yeah, so then everybody would just get a fourth of a water bottle. We have twice as many kids, but the water bottles stay the same. So they're gonna get half as much.

Pam Harris:

So it's kind of like before they had half a water bottle each and now they get half as much of that and half of the half is a fourth. Is that a thing? Okay, um, let's go back to our 500 students. So we got those 500 students. What if the situation gets even worse, and we only have 125 water bottles. We miscounted we only have 125 water bottles.

Kim Montague:

Okay, so same number of kids.

Pam Harris:

500.

Kim Montague:

500 kids, and when when they were 500 kids and 250 water bottles, they got half a water bottle. So now there are half as many water bottles as before. So they get half as much water as they got before. So instead of getting a fourth now they get an eighth. Oh, they got a half before, so sorry. They were going to get a half of a water bottle when they had 250 water bottles. But now you have half as many water bottles so they get half as much water so they get 1/4.

Pam Harris:

Half of the half is a fourth.

Kim Montague:

Right.

Pam Harris:

Okay, cool. Okay, let's see. What if I had our same 500 students? And 250 water balls. That was what we sort of started with.

Kim Montague:

Right. And that was they got a half a water bottle each.

Pam Harris:

Yeah. What if we had 250 students? And the 125 water bottles? How many? How much?

Kim Montague:

Half a water bottle per student.

Pam Harris:

How do you know?

Kim Montague:

Because I know 125 is half of 250.

Pam Harris:

Yeah. So we had the original 500...

Kim Montague:

Oh, you mean? So we have half as many students, but they get half as much water. So they're still going to get half each.

Pam Harris:

Okay, cool. What if we had 50 students, so we had we had 500 students got 250 water bottles, and then you said 250 students would get 125 water balls. Wait. How much water did they get for -

Kim Montague:

When there were 250 students, and there were 125 water bottles, they each got half a water bottle.

Pam Harris:

Did you say a fourth?

Kim Montague:

Did I say a fourth? They get a half, because they get the same as when there were 500 students, and 250 waterbottles.

Pam Harris:

How do you know it's the same?

Kim Montague:

Um, because there's half as many students and half as much water. So what they get is the same.

Pam Harris:

That seems important.

Kim Montague:

Yeah.

Pam Harris:

Is that always true? If you have half as many students and half as much water, they get the same amount of water?

Kim Montague:

As the earlier scenario? Yes.

Pam Harris:

Would that be true? If you doubled the number of students and doubled the water? Would they still get the same amount of water?

Kim Montague:

Mhmm.

Pam Harris:

Okay, so let's see. If 250 students got 125 water bottles, they each got half a water bottle. What if I had 50 students get 25 water bottles?

Kim Montague:

They're still gonna get half because and actually, I'm writing on my paper and I wish you could see, this is this is kind of a thing, but, I wrote 500 students, line 200. So over, positionally over, 500 students, for 250 water bottles, and I actually wrote equivalent or equal-to 250 students with 125 water bottles. And then I wrote 50 students to 25 water bottles, and you you found a fifth as many students. And it's a fifth as many water bottles, so that's going to be the same again.

Pam Harris:

So like from the 250, a fifth of 250 is 50. So 50 divided by five. And then from 125, water bottles, a fifth of the 125 water bottles gave you 25 water bottles. So a fifth the number of students get a fifth of the water, they still have are getting the same amount of water per student. Could you do that from the 500? Our original? We had 500 students?

Kim Montague:

Oh, yeah, that might be easier. Because you have a 10th as many students and 1/10 as much water.

Pam Harris:

Because 500 to 50 to 10, and 250 to 25 is a 10th. Okay, so what if I had only 10 students, and five water bottles?

Kim Montague:

It's still gonna be half of a water bottle each.

Pam Harris:

I mean, to me, when I look at those numbers 10 to five, it sort of screams a half.

Kim Montague:

I would have liked that problem first please.

Pam Harris:

Like if I would have said the ratio of students to water bottles is 10 to five, you could have said oh they're getting a half a water bottle each. At which point you would have said, not enough water, Pam, hydrate more. But you could have then sort of worked up and said, Well, if we had 10 students, and they got five water bottles, then 50 to 25, in fact, when I say that now, you almost could sort of look at some of these and almost see the half screaming. I wonder how many people when you heard these numbers, didn't hear the scale, and you were hearing the half sort of screaming like 500 to 250, 250 to 125, 50 to 25, and 10 to five, so that half just kind of screams out the whole time. That's kind of interesting. Why are we doing this? So there's a strategy, that if I can think about scaling the number of students and scaling the number of water bottles with the same scale factor, that I end up with an equivalent ratio of students to water. And that can be helpful when I have other random problems. So for example, Kim, another random problem. What if I asked you 244 divided by 16? Like if I had 244 students, and 16 water bottles? I don't know. That's probably not the best. This is probably not the best context to begin with at all. I probably should have come up with a better context. Sorry, everyone.

Kim Montague:

Yeah.

Pam Harris:

What are you thinking about is there something you could do?

Kim Montague:

Well, at first glance, I'm glad that we just talked about the strategy, to be perfectly honest with you, because 244 divided by 16 is kind of yucky. My palms are sweating a little bit. And here's the thing, like, I feel very comfortable taking as much time as I need. And if we need to pause, then we're just going to pause. But, um, there is something to doing math on demand that I want to make sure that I'm explaining my thinking well enough that people can follow.

Pam Harris:

I mean, you're on a podcast, give a little credit here.

Kim Montague:

I mean, yeah, okay, so alright. So I'm going to think about an equivalent problem that might be nicer. So I'm going to scale this down, divided by two.

Pam Harris:

Half as many -

Kim Montague:

Half as many, whatever's so that that's going to give me 122 to 8. 122 divided by eight.

Pam Harris:

Okay, because half of 244 is 122. And half of 16 is 8.

Kim Montague:

Um, I think you can do that again, I love that they're both divisible by two again. So I'm going to divide those both by two again, which is nice, I can half those numbers really easily. Oh, I like that. I like that a lot. Because now I have 61 divided by four, so half of 122 was 61. And half of eight was four. So now I've scaled down a couple of times, and I'm left with the problem: 61 divided by four. Does that makes sense?

Pam Harris:

Yep.

Kim Montague:

Okay. And I love that I know. 60 divided by four is 15. And one divided by four is a fourth. So at that point, I kinda changed my strategy a little bit, because I recognize 61 divided by four is 60 divided by four plus one divided by four. And so that gives me 15 and 1/4.

Pam Harris:

Cool. So you're saying you can actually use this strategy of finding equivalent ratios to solve a division problem?

Kim Montague:

Love it.

Pam Harris:

Huh, wow.

Kim Montague:

Yep.

Pam Harris:

Hey, I wonder Is that helpful in any way in a problem, like - hmm, random - how about 432 divided by five? Oh, how are you going to scale those down Kim?

Kim Montague:

Oh, you know what? I don't want to scale down because I don't - I think it's going to lead me to something odd. But what I am going to do is scale up.

Pam Harris:

What? You mean if I have twice as many water bottles, for twice as many students, they still get the same amount of water?

Kim Montague:

Yep. And so I'm going to scale up, I'm going to double the number of water bottles and double the number of students.

Pam Harris:

Random, okay.

Kim Montague:

Then that gives me 864 divided by 10, which is 86 and four tenths.

Pam Harris:

Or 86.4. Because you know, this thing about divided by 10. Because division by 10 is so brilliant. And you have a lot of numeracy and you recognize that division by five, if you scale it up to division by 10, bam! Like anything divided by 10, you can just sort of like think about that pattern of multiplying and dividing by 10 in our number system. And 864 divided by 10 becomes a 86.4. I totally just repeated what you did, sorry.

Kim Montague:

No, it's great.

Pam Harris:

Maybe that way I could cement in my own head. I wonder if you could do something similar, listeners, if you were to think about - oh, and this whole time, we should have been saying as soon as we give the problem, pause the podcast, solve it on your own. Retroactively: when you hear a problem, pause the podcast, solve it on your own, and then listen to what Kim's doing. So do that on this last problem. How about 61 divided by 2.5 or 61 divided by two and a half? Or 61 divided by two and five tenths? How would you solve that problem? So pause, pause, pause. Alright, Kim, what do you got?

Kim Montague:

Okay, I'm going to scale up again, because scaling down when it's divided by 2.5 doesn't feel like it's going to be super helpful. So I'm going to double both of those numbers and create the problem 122 divided by five.

Pam Harris:

Because double 61 is 122. Double two and a half is five.

Kim Montague:

And then I don't love that. But I'm going to scale up again, the double 122 to get 244 and double five to get 10. So now I have created the problem 244 divided by 10, which is 24.4.

Pam Harris:

Bam!

Kim Montague:

I like. Thanks for giving me problems that I like.

Pam Harris:

You're welcome.

Kim Montague:

Which really, they're not. Those problems, like, they're not, it's not like you gave me super nice ones. It's just that we have a good strategy that make them.

Pam Harris: