(jolly music) (bells and chimes) - [Children] Math Mights.

- Welcome, second grade Math Mights.

I'm so excited that you joined me for a Math Might episode.

My name is Mrs. McCartney, and we have so much planned for you today.

Let's check out what our plan is for the day.

We're going to start off doing a number talk, and then we're going to be subtracting one-digit from a two-digit number.

It's always great in math to warm up our math brain with a number talk.

If you're not familiar with the number talk, no worries, I'll help you through it.

There's three easy steps.

Our first step is that I'm going to pose a problem to you with an operation that you're already familiar with.

It's okay if you think the problem I'm showing you looks a little bit easy, because step two is not going to allow you to use pencil or paper, which means you have to think about how to add the problem, or subtract the problem without writing it down.

Our step three is for us to share out our strategies to see how we solved it.

I wonder what Math Might will be joining us today to help us with our problem.

It's my friend, DC.

(gentle music) Hey, DC.

I have him right here.

Our friend DC wears a hard hat and carries a mallet, and stands for decomposing and composing.

DC loves to add numbers when they're friendly.

In second grade right now, they're gonna be friendly decade numbers, like 10, 20, 30, 40.

If he doesn't see a friendly number, he has that hard hat on because he might take his mallet to decompose a number to make it more friendly.

I wonder what DC's problem is that he has for us today.

DC wants us to solve the problem 19 plus 13.

Can you think about that problem in your head?

19 is kind of close to one of those decade numbers that we just talked about.

Think a little bit, try not to count on with your fingers.

I bet you came up with an answer without writing it down.

Let's see what our friend Maeve thinks.

She says, "I think the answer is 32.

I decomposed 13 into one and 12, and then I added that one to the 19 to get 20, and then I did 20 plus 12 equals 32."

I don't know about you, but I think DC would be impressed with Maeve's strategy.

Let's check it out together to make sure you are understanding how she solved.

I have an abacus here.

An abacus is clear to the right and shows zero.

I like it for students that might not be able to mentally picture this in their head.

The abacus works, so we're gonna show 19, our full row is 10, and then I'm gonna do one less to show 19.

My second add in is going to go down below, which is 13, 10 and three more.

This is when DC gets upset, because we can't read this quickly.

So just like Maeve said, she took that 13 and she decomposed it into 12 and one.

I'm gonna show you here how she did it.

She put the one here and the 12.

She took that one that she pushed over and joined it with the 19, as I'm doing that here.

Do you see this nice even 20 that she made?

She took that 20, added it to the 12 that you see here, to get that answer, 32.

This is great thinking.

I know DC would love to hear this strategy.

Do you think there's another way that we might solve that problem using DC?

Let's see what our friend Keisha is thinking.

Keisha says, "I agree with Maeve's answer, but I solved it differently.

I decomposed the 19 into 12 and seven, then I added the seven to the 13 to get 20.

And I did 12 plus 20 equals 32."

That's a really great way to solve.

Let's make sure that you're understanding that and see if we can do it on the abacus again.

Here, I have our 19, then I have the 13.

When Keisha decided to do this, she looked at the 19 and realized that when she's looking down here at the 13, it only needs seven more.

So she's going to take one, two, three, four, five, six, seven and push it away.

So if you're looking at this, you can see how that 19 now is split into 12 and seven.

Next, she took the seven that she pushed and pushed it together with that 13 to make the 20 down here.

Then she added it to the 12 that you see here to get that answer of 32.

These are both great ways to solve in our number talk.

The second way does take a little bit of thinking when you're using the abacus, but it doesn't matter which number you want to make friendly.

You think about it mentally in your head, maybe you were thinking more of Maeve's way, maybe you were thinking more of Keisha's way.

Maybe you had a different way.

Either way, I know DC is proud of us for thinking about solving math mentally.

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

Our I can statement says, "I can subtract a one-digit number from a two-digit number."

We just showed how DC could help us when adding.

DC can also help us with this I can statement of the day, because he also lives in the subtraction world.

But before we learn his strategy, I want you to think about some of the things that DC does with numbers.

Let's take a look at this number, 75.

How can we decompose the number 75 into blank tens and blank ones?

If you think about that number 75, and you thought about it with place values, how many are in the tens and how many are in the ones?

Let's watch DC as he decomposes this number by place value.

He takes the number 75 and decomposes it into 70 and five.

If we look down here, I can think of that that same way by thinking about how many tens, we know that we have seven tens, and we also have five ones.

Do you think there's another way that we can decompose 75 with DC into blank tens and blank ones?

Most of my students say, "Mrs. McCartney, I just told you 75 is seven tens and five ones," but I want you to think about this for a second.

Is there a way that we can decompose 75, and not say seven tens and five ones, but be able to say it differently, where the total is still 75?

Let's see what DC thinks.

DC has an idea, where he wants to decompose 75 into 60 and 15.

Wait a minute, does 60 and 15 still equal 75?

It does.

If I took the number 75 and decomposed it into 60 and 15, which equals six tens and 15 ones, if I put that altogether, it would still equal 75.

This is something that we're gonna use today as we start to subtract with DC.

Sometimes DC can subtract using his strategy with just breaking apart the number and subtracting, but other times he needs to decompose the number differently, and we're gonna see how that works today.

Let's see.

Can we subtract this by decomposing 75 minus nine?

Let's just try right off the bat, just decomposing the number the first way we tried, since we were doing it with a number 75.

If I took it and decomposed it into 70 and five.

Boys and girls, can I do five minus nine in second grade?

Maybe when you get to middle school, you'll start to work with integers or negative numbers, but in second grade, we really can't take nine away from my five, right?

So let's take a closer look at that to see if we can decompose the 75 differently to make us be able to subtract.

If we took that 75 and decomposed it into 60 and 15, or six tens and 15 ones, we know that total still works.

I now can look at this problem and think of what is 15 minus nine?

That makes it easy.

It's six.

I didn't do anything with this 60 over here, so I'm gonna bring that 60 over and add it to my six to get 66.

So 75 minus nine equals 66.

Did that strategy make sense to you?

If you decompose by backing up a decade, right?

We were at 70 and we backed up to 60, it made it easier for us to use DC's strategy to solve the problem.

Let's see if we can play a fun game called less than 10.

The way this game works is we're going to start at the number 50, and then we're going to spin the spinner and subtract that amount to see who can get to less than 10 first.

As you can see here, I have kind of my recording sheets set up here, so we can model out the subtraction, and then I have a spinner that looks at the numbers, five, six, seven, eight, nine, and 10.

Now, if you don't have a spinner where you are, you can just take little post-it notes or pieces of paper, put them in a bag with those same numbers, shake it up, and grab out that number to play the game with a friend.

Let's see if we can use DC's strategy while we're solving for this game.

I'm gonna go ahead and spin first to see what I get.

I landed on the number 10.

Ooh, that's kind of easy.

I got lucky.

50 minus 10, oh, that's easy, is 40.

This now is my running total.

If I was playing with a friend, they would go ahead and spin and have their first spot on the recording sheet.

My arrow means that I need to transfer this total for my next spin here.

I'm ready to do my next spin now.

I wonder what number I'll land on.

I landed on the number six, so I need to take 40 and take away six.

I'm gonna go ahead and put my six in here, 40 minus six.

Now we're gonna go ahead and practice solving with DC's strategy.

You might be able to count back, since we're at such a friendly number, but let's just model it today, so we know what we're doing.

If I decompose this into 40 and zero, can I take six away from nothing?

That's not gonna work.

Let's see if we can decompose the 40 a different way so that we can subtract six.

If I were to take the 40 and decompose it into 30 and 10, I now can do 10 take away six, which might be a little bit easier, which is four.

I have that 30 over here left over that I didn't need to do anything with, so I'm gonna add it back with my 30 to get 34.

So I know 40 minus six is 34.

So this is now my running total.

I'm trying to get down to less than 10.

If I were playing with a friend, it would be their turn to go next.

Let's go ahead and do another round together to see what we can get.

Let's go ahead and spin the spinner again.

This time I landed on eight.

I need to bring my total down.

I have 34 and I need to subtract eight.

Let's try to work out that strategy together.

34 minus eight.

If I decompose this into 30 and four, and just went with it by place value.

If I have four, can you take eight away?

Not unless we wanna get a negative number, which we're not doing that in second grade.

So what should we do?

Do you remember DC's strategy that we talked about?

What happens if we back up a decade and see if we can decompose the number a different way to solve?

Let's check it out together.

This time, I'm gonna decompose the number 34 and I'm gonna back up a decade to 20 and 14.

20 and 14 still equals 34.

So if I did 14 minus eight, I know that I get six.

I haven't done anything yet with that 20, so I'm gonna add it back in with my six that I have left over to get 26.

So 34 minus eight equals 26.

Remember, now it would be my friend's turn.

The object of the game is to keep playing and subtracting until one of you gets less than 10.

The first person to get to less than 10 wins the game.

Do you think you can try out DC's strategy?

It takes a lot of mental thinking.

I know you can do it.

Now it's your turn to try to play the game that we just did with a friend, called less than 10.

Second grade Math Mights, we've had a fantastic time today from our number talk with DC and then learning DC's new fun strategy with subtraction.

I know we're onto great math.

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

(gentle music) (jolly music) - [Boy] Sis4teachers.org.

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

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

(gentle music)