(ethereal music) - [All] Math Might.

- Welcome 2nd grade Math Might, I'm so excited that you've joined us to learn more about math.

My name is Mrs McCartney, and today we have so many fun activities planned.

Let's check out our plan for the day.

Today, we're going to do a number talk and match strategies to expressions.

We have to start off our math with warming up our math brain.

Today, we're going to do a number talk.

You remember there's three easy steps, first I'm gonna pose a problem to you with an operation that you're already familiar with.

In fact, the problem might seem easy, but step two make it a little bit harder because you can't use any pencil and paper.

I want you to solve this problem mentally.

Last, we're going to share out our strategies, to see if maybe we can figure out a new way to solve our problem.

I wonder what Math Might friend is going to join us today for our number talk.

It looks like it's my friend Abracus.

(ethereal music) - Abracadabra.

- Abracus is here with me, he's one of the most magical characters in Mathville.

He loves to be sneaky, so make sure you watch out.

He likes to play magic tricks on math problems.

Oftentimes, Abracus looks at addition problems and wants to use his special strategy called compensation, by zapping one number to make it friendly.

He uses his magical one that he carries in his hand to hold the number that he's added or subtract to make that change.

In the end he has to zap it back, so he knows he's doing his addition accurately.

Let's check out what the problem is that Abracus has for us today.

He wants us to solve 19 plus 18.

Remember, Abracus likes to zap a number to make it friendly.

Look at that problem again, 19 plus 18, what do you think Abracus is going to zap?

Let's see what our friends are thinking to share their strategies with us.

I have my friend Maeve who says, "I think the answer is 37.

I added one to the 19 to make 20, then I added two to the 18 to get 20.

20 plus 20 is 40.

Then I did 40 minus three equals 37."

Wow, her strategy is really interesting.

I wonder why she did all that compensating with adding and subtracting.

Let's see if we can map out how Maeve have solved it so you can make sense of this new strategy.

Here I have the number 19 plus 18.

If you can think of Abracus zapping that 19, he's going to change it to a 20.

It's always good to make a note of how you're compensating or changing the numbers.

Maeve said she took the 18 also and added two.

So think of Abracus zapping the 18 into a 20.

20 plus 20 is pretty easy, we know that it's 40.

But Maeve said we're not done yet, we have to think about those numbers that we added and now we have to take them away.

I know we added three, so if we take the 40 and take away the three, that's how she gets the number 37.

That's a really great way to use Abracus' strategy and to use compensation.

Do you think you could solve it that way?

I wonder if there's another way we can solve it.

Let's see what my friend Keisha thinks.

Keisha solved it a different way.

She said, "I do think the answer is 37, but I added one to 19 to get 20.

Then added 20 plus 18, which equals 38.

Then I did 38 minus one equals 37."

It appears that Keisha also used compensation.

Let's check it out to make sure we understand her process.

Here I have the 19 plus the 18.

We heard that Keisha took that 19 and made it a 20.

Remember, to make that note in your head so we know we added on one.

20 plus 18 could be easy to solve, you see the 20 and the 10, which we know is 30, plus the eight is 38.

But Keisha remembered that she added one, so she had to take that 38 and take away one to get 37.

Two really great ways to solve using compensation, using Abracus' strategy.

I know he would be proud of our thinking.

Do you think that you could share this strategy with someone today, and show them how you are magical just like our friend Abracus?

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

Our I can statement says, I can match addition and subtraction strategies to expressions.

That sounds like a great plan to match addition and subtraction expressions to strategies.

Let's get our brain working by playing a game that you might be familiar with.

Our game today is called less than 10.

We wanna start this game off with a higher number than we have in the past.

We're gonna start off with 99, spin the spinner, work on our subtraction strategies, and see who can get down to 10 or less first?

I have my game board here set up with my spinner, and how we're going to go ahead and subtract?

The first person to get their total less than 10 wins.

We're gonna practice our subtraction strategies as we do this game.

I'm gonna play just as one player to review our strategies, but you could certainly play a game just like this with a friend.

Let's go ahead and spin to see how we get started.

I have seven.

I'm gonna do 99 minus seven.

Over here, we can work on our strategies.

If I do 99 minus seven, I can easily look at nine ones, minus seven ones, and know that I'm left with two.

I'm gonna keep my nine tens or my 90 there, to get the answer of 92.

In this game, this is how much you have left.

So I'm going to put it down here to start my next spin.

Spin the spinner, let's see what we get.

92 minus seven.

What strategy do you think that we could use?

We could use Springling, we could decompose the seven and solve using DC.

We have lots of different ways we can solve it.

I think I'm gonna use my friend DC, and decompose the seven into two and five.

I know two minus two is zero.

So that I'm left with just 90 minus five.

I can skip count pretty well, and I've already subtracted that too, so I know I just need to subtract the five, which is going to give me 85.

So 92 minus seven gives me 85.

It's time for me to spin the spinner to see if we can keep taking that difference and bringing it down closest to 10.

I ended up spinning nine, so I'm bringing my total down here doing 85 minus 9, 85 minus nine.

How should I solve that problem?

I could do it in so many different ways.

I don't know, I think I'm gonna try it out with Springling and hop some really big hops.

I'm gonna start off my number line at nine, and I'm going to end it at 85.

I know one more gets me to 10.

That's easy and I'm gonna go wild and hop all the way to the 80.

If I go all the way to the 80, I know that that is 70.

80 to five is simple.

I know that that's five, Springling likes to add up those to see how far she hopped, 70, 75, 76.

I know 85 minus nine is 76.

Let's keep going and spin again.

Ooh, this time I got 10, 76 minus 10.

How can I think of that?

Let's just decompose the 76 into 70 and six.

Let's concentrate on the tens there and get rid of 10.

I know if I did 70 minus 10, I would be left with 60 or six tens.

Let's bring that six over to get 66.

So we have 66.

Let's go ahead and fill that in.

We had 76, which is what we are left with.

We spun on the spinner a 10, and I know my remainder leftover was 66.

This game would continue to be played until the first person has less than 10.

The cool thing boys and girls that I want you to realize, is by now in 2nd grade you should have multiple ways to do subtraction.

You can think about the problem and decide which strategy might be more efficient for you to use.

I know our characters in Mathville are proud of our work with subtraction.

Let's play a fun game to see if we can match up Maeve and Keisha's strategies to addition expressions.

In this game, they're only using DC or Value Pak strategy.

This is gonna be a bit challenging because we've erased the strategy that they've used, and we've only did the last part or the last step, in the process of addition.

We have to figure out which strategy they use to match that expression.

Let's take a look at our first expression, it says 35 plus 42.

If I were to look at this, 30 plus 30 is 60, would there be any reason I would be adding these and get 60?

No, if I use number sense and think about estimating, I know that that sum is way too high.

Let's check out this, 70 plus seven.

I wonder how they could have gotten this concept.

If I were to think of DC strategy, I really would have a hard time trying to make 35 friendly or 42.

But if I were to add my tens and tens and ones and ones, like our friend Value Pak, this might match.

Let's see if we can map this strategy out to see if it matches that final step.

I'm gonna write down 35 plus 42.

Our friend Value Pak likes to decompose by place value to add.

I can tell by the girls' strategies that they ended up adding the 30 and 40 together to get 70, and the five and the two together to get the seven, to get the total of 77.

I think that we need to put Value Pak next to this problem because the last step, showed me what the girls were doing with this strategy.

Do you get the idea?

Let's try another one together.

Our next problem says 27 plus 33.

I'm kind of looking over here and realizing that there's no way it would be 80 plus 11 that you would end up with your last step in the strategy, when you're looking at the numbers 27 and 33.

It's really important math to use estimation or our number sense to make sense of things.

If I look down here, I also see the same problem, 61 and 30 to equal 91, that doesn't seem quite possible either, because we know that this wouldn't total that.

If I look up here I can think a little bit more.

If I thought of this with Value Pak, I would have 20 and 30 which equals 50, seven and three equals 10, which none of those show.

So, I'm wondering if the girls on this one, we're using DC?

Let's give it a try.

27 plus 33.

If I were to decompose the 33 into three and 30, I know I would make a 30 here which is shown here.

Plus the 30 gives me 60.

We need to put a DC on this one.

Great job trying to think about that by using some estimation in number sense, and then kind of thinking about this backwards.

Normally, you're given the expression and then you solve with a strategy.

It's a little bit more complex to figure out what strategy they use, but if we look at it carefully and use the number sense, I think it's something that you can do.

Now, it's your turn to play a game called test your luck challenge level.

This challenge level is going to involve addition and subtraction, like we've done today on our show.

2nd grade Math Mights, we've had a great time today from our number talk with Abracus, and then being able to match expressions to strategies.

I sure hope I see you soon on another Math Mights episode.

(ethereal music) - [Instructor] Sis4teachers.org.

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

- [Narrator] The Michigan Learning Channel is made possible with funding from the Michigan Department of Education, the state of Michigan, and by viewers like you.