(upbeat music) - [Voice-Over] Math Mights!

- Welcome second grade Math Mights.

I'm so excited that you've joined me to learn more about math.

My name is Mrs. McCartney, and we have all kinds of great activities planned for today.

Today, we're going to start off with a Number Talk, and then we're going to add and subtract within a hundred.

Let's start off warming up our brain with a Number Talk.

A Number Talk has three simple steps.

Our first step is I'm going to pose a problem that today you might find is kind of easy, but don't worry because step two will stump you.

You can't use any pencil or paper, and I don't want you to use your fingers.

Let's see if you can solve it using mental strategies.

Our step three is for us to share out how we solved it and see if maybe we can learn a new way to solve our problem.

Let's see what Math Might is going to join us today for our Number Talk.

It's my friend Abacus.

He's one of the most magical Math Mights.

I have it right here.

Abacus has a magic hat and carries a wand.

He loves to use the strategy in addition called compensation.

You might've learned this strategy in first grade and know it as doubles plus one or doubles minus one.

Our friend Abacus likes to zap numbers to temporarily play a magic trick on them to change them to a way that he can solve it easier.

His wand is magical because it holds that number in there.

So if he takes away two, he has to zap it and add the two back.

Let's check out first what the problem is that our friend Abacus wants us to solve.

Abacus wants us to solve the problem nine plus eight.

Remember Abacus wants to zap one of the numbers to change it temporarily.

To give you some mental help, I have built it on a double 10 frame map.

I have the nine and I have the eight.

I think Abacus might want to zap one of these numbers to temporarily change it, to make the addition easier.

Do you think you have an answer in your head for nine plus eight?

Let's see what our friends are thinking.

Today, I have with me Maeve and Maeve thinks the is 17.

She added one to the eight to make it nine plus nine, which is 18.

Then she took that one off to make it 17.

Are you familiar with the double fact that Maeve is talking about, nine plus nine?

Let's take a closer look to see if we can solve it together.

Here I have the nine plus eight.

It sounds like she took Abacus and zapped the wand to see if she could add one more right here so she could make it nine and nine.

If I add the one more, it temporarily changes it.

So let's make a note of that change.

She took the eight, I'm going to kind of draw an arrow and I'm going to put plus one.

I'm going to put it in a circle because we know Abacus is kind of holding that in the wand.

When we look at nine plus nine, you might be able to see that it's 18 because you know the doubles fact.

Now this was temporary or a part to compensate to help us, so we need to take it back off, which is why she did 18 minus one gives us 17.

This was a really great way to use our friend Abacus's strategy.

Let's see if there's another way that we might be able to solve this with my friend Kiesha.

Our friend Kiesha said, I think the answer is 17.

I subtracted one from the nine to make it eight plus eight.

Then that equals 16.

Last she decided to add that one back to make it 17.

Do you think you're making sense of Kiesha strategy?

Let's check it out together to make sure we know how it works.

Here, she saw the nine and the eight, this time she's zapped with Abacus' wand, the one to take it off, to make it eight plus eight.

If we make a note of that, I'll kind of draw an arrow down and remind us that we took away one so that we don't forget to add it back at the end.

We know eight plus eight equals 16.

If we take that one and we add it back, we know that it's 17.

So I'm going to put that number back the way it is.

So you can see that total.

Let's check out our, "I Can" Statement of the Day.

our "I Can" Statement says I can add and subtract within 100 in more than one way.

We're going to be using those base 10 blocks or the idea of place value to help us with addition.

I have to have one of my Math Might friends come help us.

Let's see who it is.

It's my friend DC.

DC loves to solve addition by decomposing and composing.

But how does that relate to place value?

Today, we're going to look at that.

The idea that DC likes to do is he likes to decompose numbers to be able to make a friendly number and the place value part of it is being able to think of things that are in decade numbers like 10, 20, 30.

Sometimes DC strategy, especially as we're adding numbers up to sums to a hundred in second grade gets tricky, so DC is going to help us by using the Abacus.

Let's check out the problem that he wants us to work on today.

We first want to see if we can solve this problem in two ways, it says 27 plus 47.

Let's see first if we can solve it one way.

I have an Abacus here.

If you can't figure out how to do DC Strategy right away, you can use an Abacus like this.

The beads are clear to the right.

I'm going to show the first add in.

Now DC doesn't like it when numbers aren't friendly.

27 is an unfriendly number, but it's really close to a decade number, a 10, 20, 30, 40, let's look at the Abacus.

If I'm at 27, it is so close boys and girls to get us to that 30.

So if we recognize the 27 only needs three more to get to that decade, we can go over and take that 47 and decompose it.

If you're wondering how many that 27 needs, you know they only need three.

So if I took the three out, we know that we can make the 30.

Some students in second grade, get stumped though, if I have 47 and I took three out, how many is left?

Well, let's take a look at that on the Abacus.

If I were to show 10, 20, 30, 40, and then seven, and I took three away, you can see quickly that you see that there are 44 left.

You can always check to see if 44 plus three equals 47.

Now that we've done this, we can put this together in that friendly 30, and then we can add 44 to it.

Again.

If you're struggling with adding this, we can use the Abacus.

Let's do our 30, 10, 20 30, and then let's add on our 44, 10, 20, 30, 44.

I can read the beads 10, 20, 30, 40, 50, 60, 74 to get the answer.

We know that 27 plus 47 equals 74.

Great job using DC's strategy.

Do you think there's a different way that we could solve this problem?

Hey, look boys and girls, it's my friend Value Pak.

They want to see if we can solve using their strategy.

I have my friend Value Pak here.

In addition, they like to use the strategy called Partial Sums.

They like to decompose by place value in the tens and the ones to help us solve addition within a hundred.

Let's check out what problem they have for us today.

Oh yeah, it's that same problem we just did!

27 plus 47.

But this time we're going to solve with Value Pak.

I have the problem here in front of me, 27 plus 47.

They click together and make a value.

But when we decompose by place value, remember I didn't say decompose to make a friendly number, decompose by place value, which means tens and ones.

If I took that 27 and broke it apart, it would be 20 and seven.

If I took the 47 and decomposed it by place value, it would be 40 and seven.

Value Pak likes to add it's tens and then it's ones, 20 plus 40 is 60, seven plus seven is 14.

Put that together, and I know that I have 74.

Two different ways to solve the same problem, Value Pak and DC.

Do you think you can share with someone how to do both of those strategies?

Now that we're getting good at this addition, let's check out the subtraction.

Do you think that you can solve with Springling for this problem?

55 minus 27.

Our friend Springling love subtraction on the open number line.

She was born with fancy eyelashes and fluffy fur along with this coily tail.

She likes to hop to find the distance between two numbers using friendly hops.

Let's check out her strategy.

We have 55 minus 27.

I have this on the open number line.

We're going to find the distance between two numbers.

We're going to start at 27 and hop to the friendly number 30.

When Springling hops, she likes to keep track of how far she's gone.

She doesn't like it when second graders hop 1, 1, 1, because that flattens her fur.

So we know the distance between 27 and 30 is three.

If we hop, we can go from 30 to 40, which we know is 10.

And from 40 to 50, which we know is another 10.

And then we can have from 50 to 55, which is five.

If you're able to hop from 30 all the way to 50, and you know it's 20, you can do that all on your own.

Let's see how far Springling has hopped.

10 plus 10 is 20 plus five more is 25 and three more makes it 28.

When we add this distance together, it tells us how far she's hopped.

So 55 minus 27 equals 28.

That was a great way to solve that problem with Springling, but she asked if we could solve it more than one way.

Let's see what character she wants us to use.

Here, she wants us to see if we can use DC to subtract that second way with 55 minus 27.

Now with DC, we know in addition, he likes to decompose and compose to make numbers friendlier.

When he's using subtraction, he likes to decompose and try to rename numbers.

So if we were to try to solve this, I know I could do 50 minus 20, but if I had five ones and I wanted to take away seven, I can't do that.

So DC likes to use his strategy of decomposing the number by backing up a decade to make 55, 40 and 15.

40 and 15 when you put it together, still does equal 55.

This is a great thing to learn before you learn how to do the traditional method with T-pops.

We're going to decompose the 27 into 20 and seven.

40 minus 20 is 20.

15 minus seven, we know is eight.

If we add 20 plus eight together, we know we have the number 28.

Using DC strategy can be challenging sometimes for second graders, but I want you to see if you can back up a decade and rename that number into tens and ones to help you subtract a lot easier.

I can't believe we solve that subtraction with two different Math Might friends.

It's your turn to do a Math Might Challenge to see if you can add and subtract with all the characters that we featured in today's show.

Second grade Math Mights, I'd had an awesome time hanging out with you today from our Number Talk that we did with our magical character Abacus.

And then we worked with strategies using DC for addition and subtraction, Springling for subtraction, and then Value Pak for addition.

Wow, you have to show somebody today how many strategies you can do with our math Phil friends.

I can't wait to see you on another episode soon.

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- [Voice-Over] Changing the way you think about math.

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