(playful music) (magical music) (glass squeaking) - [Children] Math Mights!

- Welcome, second grade Math Mights!

My name's Mrs. McCartney.

I'm so excited that you've joined us today for an episode to learn more about math.

Today we're gonna be doing a number talk, and then we're going to add and subtract three digit numbers.

Let's start off first by warming up our brain with a number talk.

Remember, a number talk has three easy steps.

Let's see what Math Might friend is here to help us with our number talk today.

(playful music) (explosion booming) It's our friend DC!

DC is a Math Might friend that wears a hard hat and carries a mallet.

DC loves to solve problems with decomposing and composing.

He walks around Mathville trying to find problems that he might want to decompose to make friendlier, into tens or even decade numbers.

He likes to smash them to find an easier way to add it by composing it back together.

I want you to think about DC's strategy as you check out this problem today.

DC wants us to solve 189 + 21.

Let's check out to see how our friends solved it.

Our friend Shannon said, "I think the answer is 210.

I decomposed the 21 into 11 and ten.

I added the 11 to 189 to get 200, then I added the 10 to get 210."

Shannon gave a great explanation for how she added with DC, but I'm not sure I totally understand it without acting it out, so let's do that together.

She saw 189 + 21, and decided to decompose the 21 into 11 and ten.

Why do you suppose that Shannon did that?

Why would she take the number 21 and break it into 11 and ten?

Remember, DC is looking for that friendly decade number in this case, even a number that's in a nice even hundreds.

So if we added this 11 to the 189, I see what she was thinking.

She wanted to make that friendly number 200, and then add in the 10 to get the total of 210.

189 + 21 DC's way equals 210, which is a lot easier to look at when you decompose the numbers.

Did you solve the problem the way that Shannon did, or did you solve it a different way?

Is there another way to solve the problem with DC?

Let's check out to see how Kathleen solved it.

Kathleen said, "I like the way Shannon did it.

I think you could also solve it with decomposing 189 into 79 and 110.

Then add the 79 to the 21 to get 100.

Then add the 100 to the 110 to get 210.

Wow, that was a completely different way of solving it using the same strategy, but decomposing a different number.

Let's check that out together.

Kathleen took the 189 + 21, and was trying to get that 21 to be a friendly hundred.

That's a little bit harder to do, but it's great to be able to expand your thinking on how you can solve problems different ways, because you're using great number sense.

Here she decided to decompose the 189 into 79 and 110.

110 and 79 together equal 189.

She composed the 21 with the 79 to get that nice friendly 100, and added it to the other 110 to get the total of 210.

Those were both great ways to use DC to solve with addition.

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

Our I can statement is I can add and subtract three-digit numbers.

Mia and Lin were asked to find the value of 500 minus 387.

On the screen, we can see how Mia decided to answer the problem.

It looks like she used Springling as one of her strategies to help her.

Let's walk through the way Mia solved it, but first we have to call on my favorite Math Might friend, Springling.

Hey, Springling, can you come help us?

(playful music) Springling is a Math Might that was born with fancy eyelashes and fluffy fur.

She has a large coily tail and loves to count up or back on the open number line.

Mia used that strategy with this problem.

For Mia's way, I decided to already put the number line out as she had it trying to find the distance between 387 and 500.

She started off first by hopping a large chunk of 100 to bring her from 387 to 487.

Next, she decided to hop 10 up the number line to get her to 497.

She's getting a lot closer to that 500.

She only needs to hop three more to get to that 500.

The distance between the two numbers is 100 + 10 + 3, which would give us 113.

I think by now you're a pro with solving subtraction problems with Springling.

I love the way Mia solved that problem.

Let's check out the way Lin solved it a different way.

Lynn uses a strategy called compensation, where she's shifting the number line to subtract.

With this strategy, we have one of our new friends from the Math Mights, Minni and Subbi!

(playful music) Minni and Subbi are a really fun character in Mathville.

In fact, they're twins that were born with an adjoining tail.

Minni stands for minuend and Subbi stands for subtrahend, the two terms that we use a lot in subtraction.

They were born with a coily tail, and they really hate it when their tail drags in the mud.

But when one walks one way, the other must follow, because their tail is attached.

Just like all sisters and brothers, they fight a lot, and so their parents have said, "You need to always stick a tail's width apart."

So we have to make sure when we're using Minni and Subbi that we're always shifting the number line the same distance.

Let's check out the way Lin solved this together so you can understand compensation in subtraction.

Here we have 500 minus 387.

Minni and Subbi are not fans of regrouping.

Although they're good friends with T-Pops in Mathville, they really dislike zeros, and they find to subtract without regrouping makes it a little bit more efficient.

Here we have 500 minus 387.

Minni is the character that has the baseball cap on, and she's nicknamed after the minuend.

Subbi is the second number in subtraction, and her nickname is from the word subtrahend.

If we were to look at 500 minus 387, Minni and Subbi don't like to subtract across the zeros because they have to regroup.

They like T-Pops in the traditional way, but they like to shift the number line.

So let's take a look on the number line to see the distance between these two numbers.

If I were to take a piece of string, we know the original distance is from here to here.

However, they don't want to have to regroup, so they're going to shift the number line back to 499, which is taking off one.

The original distance between the two numbers is this row here.

If I were to do it this way, we can see it's a different distance.

We know the number line has moved, which is making their tail saggy.

Let's see what that looks like.

Here's the original distance.

If Subbi or Minni decides to move, the other sister has to move.

So in this case, our friend at the top here decided to take off one and go back to 499.

When we do that, what happens to Minni and Subbi's tail?

Well, it gets droopy.

They're not liking that.

They want to keep the same distance.

So if we took away one on the minuend, we have to take away one on the subtrahend to make sure that we're shifting the number line the same distance.

So now the distance between the two new numbers we know is the same.

Let's test it out.

387, if we pick this up and move it, the distance is going to stay the same.

Now if we were to subtract this one, we know that we could subtract this a whole lot easier.

This problem no longer involves regrouping.

Would you rather solve the problem by going across all the zeros, or just shift it one in both directions by making the minuend and the subtrahend shift so you could solve 499 minus 386?

If I were to solve this, I know that I would end up with the same answer that we did when we solved it with Springling, 113.

This strategy is a little bit tricky when you start to learn it in second grade.

A lot of second graders love to try this strategy instead of regrouping across the zeros with T-Pops.

It's a great challenge to try it out on your own to see which one you like better.

Let's see what Shannon and Kathleen think about the two strategies from Springling or using Minni and Subbi with compensation with shifting the number line.

Shannon says, "I wonder if Minni and Subbi's strategy will always work when subtracting across the zeros?"

Kathleen says, "Oh my goodness, I can't wait to tell someone about this amazing strategy!"

Those are really great thoughts that Shannon and Kathleen had about solving with Minni and Subbi.

Let's explore Shannon's thoughts a little bit more.

She was asking, could we do this with all problems with zeros?

Let's take a look.

We have 600 minus 239.

Over here, I did it the way T-Pop does.

Probably the way your mom and dad have learned, you probably know how to go across the zeros.

I don't know about you, but when we're using T-Pops going across the zeros, sometimes I get mixed up, and it's a little bit more complicated than doing it with Minni and Subbi.

If we did it with Minni and Subbi, we can look at the distance between the two numbers, which is from here to here.

We could back up the number one by subtracting one.

When we use our string, we can see that the girls are gonna be sad because their tail is dragging in the mud.

So we need to back the number line up so that we can put it at 238, which is also subtracting.

So we shifted both numbers so we can see that distance.

So we took the 600 and we took away one to make it 599, and then we took away one from the subtrahend, 239, to make it 238.

We were to subtract that, it would be pretty quick, no regrouping necessary.

When you look at both of those strategies, which one is the car method that might take us on a detour if we do any arithmetic wrong, or which one is the jet plane?

We can see the car method here.

Although you might like doing this method, some students might find this to be a little bit slower than just quickly shifting the number line so that we now have a non-regroup, which is what Minni and Subbi absolutely love.

Andre and Tyler were asked to find the value of 427 + 351.

Here we can see Andre's work, and then we can kind of look closer at Tyler's work.

Both students found the same answer, but looks like they solved a different way.

I know the friend that they used, our friend Value Pak.

(playful music) They like to decompose by place value.

When the Value Pak is clicked together, they show the value of what they are together.

When they click apart or separate, their value is displayed on their belly.

This is a really great strategy called partial sums when solving three digits plus three digits.

Here is how Andre solved, and here is how Tyler solved.

They both got 778, but when Tyler did it, he decided to do kind of a branching number bonding system by decomposing by place value.

He used the colors to show how he decomposed and then added together.

Another way that second graders might like to use after they get really good at visually showing it this way is they can decompose by place value by doing 400 + 20 + 7 to decompose 427, and take 351 and decompose into 350 and one.

Then add the hundreds, add the tens, and add the ones, similar to how Andre did it.

Two different ways that you could show your work for how to solve with Value Pak using partial sums.

That's a fun strategy to use in second grade to help you with larger addition, or you could also use T-Pops or another strategy as well.

Now it's your turn to solve problems with Springling and Minni and Subbi, like we did today.

Second grade Math Mights, I've had a fantastic time hanging out with you today, from our number talk with our friend DC, learning about Minni and Subbi and that really cool strategy, and also talking about Value Pak and of course, Springling.

I want you to share that strategy of Minni and Subbi with someone else so that they can see how easy subtraction can be across the zeros with compensation.

I can't wait to see you on another Math Might show soon.

(playful music) (playful whistling music) - [Child] Sis4teachers.org.

- [Child] Changing the way you think about math.

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