(upbeat music) - Math Mights!

- Welcome.

Second grade Math Mights.

I'm so excited that you've joined us to learn math.

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

Today we're going to do a number talk and look at differences with two digit numbers.

To start off math it's always best if we warm up our brain.

Today we're going to warm up our brain with a number talk.

If you remember, a number talk has three easy steps.

The first step is we're going to pose a problem to you.

It's probably an operation that you're already familiar with and you might even find the problem looks easy.

But that brings us to step two, you're not allowed to use any pencil and paper.

You're going to solve it mentally in your head.

And then step three, we're going to share out our strategies to see if you can learn a new strategy from one of our friends.

I wonder who's going to help us with our number talk today.

It's my friend, DC.

(upbeat music) He loves to decompose and compose.

That's what his DC stands for.

He likes to find friendly numbers when he's adding, like decade numbers in second grade 10, 20, 30, 40.

If he doesn't see a friendly number, he uses his mallet to decompose the number, to find that friendly number to add easier.

Let's see what our problem is today that DC has for us.

It says 27 plus 18.

Can you think about which of those numbers DC might want you to decompose to make a friendly number?

Of course we could count up by ones, but these problems are getting harder to do mentally, which is why our brain has been practicing using this strategy.

Let's see what my friend Maeve is thinking.

She said, I think the answer is 45.

I decomposed the 18 into 3 and 15.

Then I added the 3 to the 27 to get 30.

Then I did 30 plus 15 equals 45.

Do you understand Maeve's strategy?

Did you solve it similar to Maeve?

Let's check it out together so we can make sense of it as a group.

Here I have the number 27 plus 18.

I'm going to go ahead and build 27 on my abacus.

The abacus is grouped into tens and ones, and it's very easy to see without counting every bead.

On the next line I'm going to build our second add in, which is 18.

DC cannot look at this and add because it's all messed up.

He likes things to be friendly, which is why Maeve decided to decompose the numbers to solve.

She decided to decompose the 18 into 3 and 15.

Here's our 18.

We know what only takes 3 more to make this 27, a friendly 30.

So she decomposed that 18 into 15 and 3 took that 3 and brought it back with the 27 to get this nice friendly number 30.

Then she just needed to add in the 15.

We know that 30 plus 15 is 45.

We also can use the tool, the abacus to see that answer quickly.

Did you make sense of this strategy using the abacus and the way that Maeve solved it?

Let's check out our 'I can' statement for the day.

Our 'I can' statement says 'I can find the difference with 2-digit numbers.'

Did you hear the word difference in our 'I can' statement?

You know what that means?

We have to meet another Math Might friend that I know you're going to love.

It's my friend Springling.

Here she comes now.

(music) She was born with fancy eyelashes, fluffy fur, and a coily tail.

She loves to do subtraction by counting up or back on an open number line.

She's very competitive and she likes to hop really far because she likes to find out the distance between two numbers.

In fact, she has a secret.

She really likes adding and likes to keep track of her hops.

Sprinkling doesn't like it when second graders hop one, one, one because you know what, it flattens her fur.

Today we're going to be using Springling's strategy.

And I can't wait for you to give it a try.

Let's see first what she wants us to do.

Springling wants to know, what does subtraction mean?

I want you to think about this.

What does subtraction mean to you?

Let's see what my friends Maeve and Keisha are thinking.

Our friend Maeve says subtraction means take away.

Of course, when you're thinking of it in kindergarten or first grade, you're thinking of something and you're taking it away, which means to subtract.

That's a really great idea, Maeve.

Keisha has a different idea.

Keisha says, I think subtraction means the distance between two numbers on a number line.

She's also correct.

In second grade, we're not going to be taking away one, two or three at a time like we did in kindergarten and first grade.

We want to start to think of numbers on a number line.

And it's looking at the distance between two numbers on a number line.

Springling wants to know what is a friendly number or a decade number.

This is really important to know when you're doing Springling's strategy.

Let's take a look at this number line because I know Springling does not want us hopping one, one, one.

She's going to want us to hop on friendly numbers.

Here I have a number line.

If you need help in second grade, I love making students number lines and maybe circling the decade or friendly numbers.

On this number line, if I wanted to look at the distance between 9 and 15 instead of hopping 1, 2, 3, 4, 5, 6, you could hop to a friendly number, which is 1, and then hop all the way to the end to get to 15, which would be 5.

Our friendly numbers are on the number line, sometimes they're decade numbers.

But as we start to expand our knowledge in subtraction, you might hop on a number, a whole group of 10.

The ideas for this strategy are really limitless.

I think today we should practice so that we can share with our friends how we can use Springling's strategy to subtract.

She wants us to count up or back on the open number line.

Our problem is 46 minus 28.

Do you see how, when you look at this problem, when we're talking about larger numbers, it would take us a long time to count out 46 of something and then subtract the 28.

This time we're going to look at the distance between the two numbers and we can see if you might be able to solve this a little bit easier.

The way we set this up with Springling is we want to look at the distance.

Now, if I was making a number line, you could make all those lines, but Mrs. McCartney loves to create what's called an open number line.

An open number line can start and stop wherever you want it to.

If you're struggling with this concept, go ahead and get a number line and match it up underneath yours to help you figure out where the numbers will go.

We know if we start at the number line and I'm counting 1, 2, 3, 4, 5, I'm going to come to the number 28 first.

So I'm going to go ahead and put 28 here.

And at the end, I'm going to put 46.

This is how we're thinking of the distance between two numbers.

Now, if I wanted to, I could hop 29, 30, but Springling is saying no, no, no, no.

We don't want to do that because we could just hop to that friendly decade number.

So if we're at 28, we know that our friendly decade number is going to be 30.

Now remember, Springling has this coily tail, right here, and she wants to keep track of the distance.

I love to tell Springling, hop Springling, hop!

How far did she go?

Between 28 and 30, we know it's two.

Mrs. McCartney likes to keep track of Springlings hops because she's very competitive and wants to see how far she went.

Now I'm at the number 30.

Should I do?

31, 32, 33.

No, that's going to flatten her fur.

Some kids might say, well why don't we go to 35?

We could.

I want you to know the way that you hop on the number line, is your choice.

I would like to hop on friendly decade numbers, because it helps me to think of the math quicker.

But if you need to slow down and go to smaller increments and then build up, some of my second graders go by two, and then by five, and they finally feel comfortable with going with 10.

I think today we're going to hop 10.

Let's check it out.

If I was at the 30 and I wanted to hop to the next decade of 40, I would tell Springling hop, Springling hop.

This is easy for me to solve because I know she went 10.

I have to get to that end number of 46.

If I hop from 40 to 46, I know how far it is.

Hop Springling, hop.

I know that it's six.

Now, what is the answer?

Is the answer 46?

Wait a minute.

Let's go back and talk about what Springling was doing.

She likes to hop on that number line to find the distance.

She's very competitive and wants to know how far she hopped.

So let's add these numbers together.

10, 6 is 16, 17, 18.

We know that the distance between the 28 and the 46 was 18, therefore 46 minus 28 is 18.

Are you understanding Springling's strategy?

It's kind of fun isn't it?

I know she'd be really proud of you.

I wonder now if you can use another Math Might to subtract with that same problem.

Let's see who it is.

It's my friend DC.

He wants to see if we can subtract using his strategy.

Now DC, in addition likes to make those friendly numbers, but in subtraction, he does things a little bit differently.

Let's take a look at this 46 minus 28.

Let's just say you wanted DC to go ahead and decompose by place value.

Sometimes in second grade, we have to check to see if we can do it this way and that's totally fine.

I can do 40 minus 20, but can I do 6 minus 8?

If I have 6, can you take away 8?

You can, it would get a negative number, but we're not going to do that in second grade.

We want to be able to use our number sense to help us.

So in this case, if I decompose the numbers just the way they are, I won't be able to subtract very well.

So I think what I'm going to do is I'm going to think of how I can decompose this number differently.

I'm going to back up a decade and just rename this 30 and 16.

I'm going to leave my 28 just the way I have it.

Can I do 30 minus 20?

Absolutely.

I know that leaves me with 10.

If I do 16 minus 8, I know that will give me 8.

This is what I have left over.

So I'm going to add it together to get 18.

30 and 16 equals the 46.

Just think of it about backing up the decade so that we now can subtract easier.

DC's strategy in second grade sometimes takes a lot of practice.

I encourage students to really try to use number sense to solve it this way before being pushed to do it in the traditional way with T-Pops.

There's plenty of time for T-Pops, but if you could prove to someone that you could solve it with DC, I bet they might be pretty impressed.

Taking a look at DC's strategy and Springling's, which strategy do you like to use?

Do you like to use decomposing and composing like we did with DC?

Or did you like to count up or back on the number line like we did with Springling?

They do have different ways, but at the end we do get the same answer.

DC decomposed by backing up a decade, remembering that 30 and 16 still does equal 46.

We ended up subtracting each of the parts to get the answer of 18.

For Springling, we hopped on that number line to see the distance between the two numbers.

Wouldn't it amazing if you could share with someone today that you know how to subtract two different ways using our Math Might friends.

Now it's your turn to be able to use the strategies we learned today in a game called Race to Subtract with DC and Springling.

Second grade Math Mights, We've had a blast today.

From our number talk to now being able to subtract two digit numbers, two different ways.

I hope you join us for another Math Might episode soon.

(whistle music) - SIS4teachers.org changing the way you think about math.

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