(upbeat music) ("Happy Birthday") - Hey, everybody, welcome back to "Math Time."

I'm working on my performance because pretty soon my wife's birthday is coming up and I want it to be a special day so I'm working on my song, trying to learn it on the harmonica, and you know, I need some help solving a problem because I want it to be just right.

So can you help me out?

I hope you can and I think you can.

So here's what we've got.

I've got a cake right here, right?

And so, I've got a cake for her birthday, and as I'm getting this, I'm thinking, okay, this is great, this looks good, but I wanna get some candles to add to the cake.

And so I went to the store and I was looking at those candles and I said, okay, I know that I need 30 candles 'cause she's turning 30.

Shh, don't tell anybody.

But the candles, they came in packs of five.

Okay, so packs of five, I'm trying to get 30 candles, how am I gonna do this?

Well, let's think about the number 30.

I know that the number 30 can be broken up into groups of 10, right?

10, 20, 30.

But can it be broken up into groups of five 'cause it comes in packs of five?

Well, sure it can.

Let's count by fives and see if we get to 30.

Five, 10, 15, 20, 25, 30.

And we did it.

So I know that I can get just the right amount of candles with packs of five, but how many packs do I need to buy?

Now I've gotta figure that out.

Well, let's go back to our counting by fives and this time let's keep track of how many groups of five it takes to get to 30.

Okay, five, 10, 15, 20, 25, 30.

It was six packs.

Okay, so that means if I buy six packs of candles, if they come in these packs of five, I'm gonna have exactly 30, which will be perfect for her birthday.

All right, this is great.

We're going good.

Now, let's think about it's probably gonna cost me some money isn't it?

Okay, let me look at my pocket.

Let's see.

Okay, I've got a $10 bill.

Now, each pack of five candles cost $1.

So do I have enough?

Well, yeah, sure, right?

A dollar per pack, that's gonna be six packs of candles, that's what we figured out, and that's gonna be $6.

Okay, so 10 is greater than six, so I'm good to go, but shouldn't I get some money back?

I think I should get some money back.

So let's figure that out.

All right.

$10, it's gonna cost $6, we can use subtraction to figure this out.

We can make an equation out of it too because I know that 10 minus six equals how much money I should receive, right?

Because I'm giving them 10, they're taking six, and then they're going to give me back whatever's left.

All right, so how much would that be?

Well, we could figure that out in a few different ways.

One, we could just count by ones backwards from 10, right?

And we could do that six times.

Maybe you use a number line, maybe you draw it out, or you could think about how you can break apart the number six.

I know that six can be broken into a five and a one.

So 10 take away five I know is five.

Now we have to take away one more and we're down to $4.

$4 is how much I should get back if I give them 10.

Oh man, we're doing great.

We're solving problems, this is awesome.

There's one more problem I've gotta solve.

See this cake?

It's in one giant piece, but there's gonna be me, my wife, and our two girls, and so there's gonna be four people total and I want all of us to have an equal share of this cake.

So can I cut it into equal shares?

Well, sure I can.

But what shape is it?

I know that this is a square.

I have a square cake, so how can we cut a square into four equal parts?

Well, I know if I draw a line or cut a line right down the middle here going vertical, that's that up and down way, okay?

So if I cut this cake vertically, I can cut it into two equal halves.

All right, that's that's awesome, that's a start, but I want four equal pieces, not just two, so I've gotta make another cut and this time I'm gonna make a cut, let's go straight across horizontally.

Horizontal is that side to side, okay?

So now that I've cut one all the way down the middle vertically, up and down, and one horizontally, side to side, how many pieces do we have?

We've got one, two, three, four pieces.

So that's how I'm gonna have to cut this cake.

I mean, you guys are doing great, thank you so much for helping me out.

Now, I've gotta get back to practicing because I don't know if I'm ready to do my song.

Should we, let's hear.

("Happy Birthday") Ugh, I think I need some more practice, but we're getting closer.

I'm almost there.

I'll keep practicing and you keep learning, you keep problem solving and figuring out different things.

We worked on equal shares today, we talked about subtraction, all these different problems, so you keep working on those and we'll see you again real soon.