(upbeat music) - [Children] Math Mights!

- Welcome second grade Math Mights.

My name's Mrs. McCartney.

Thanks so much for joining us today to learn about Math.

Here is our plan for today.

Today we're gonna do a Mystery Math Mistake, and then we're going to do problems with money.

Let's start off first with warming up our brain with a Mystery Math Mistake for my friends, The Math Mights.

Oh no, what's happened to all of our Math Mights friends?

They're all turned around and dizzy, and have all their strategies messed up.

During our Mystery Math Mistake, one of our Math Might friends is going to join us.

And your job is to be a Math detective.

To use your magnifying glass to look up close the strategy we're doing to see if you can find the Mystery Math Mistake.

Our friend T-Pops has this problem for us, 85 minus 29.

T-Pops here is upside down and all mixed up with his strategy.

We're hoping we can turn him around and find some ideas as to where that Mystery Math Mistake is.

For 85 minus 29, we're gonna start off in the ones column.

I know that I cannot take nine away from five.

So I'm gonna go ahead and rename 85, seven and 15.

Seven tens that is, and 15 ones, even though he's upside down he's still watching.

15 minus nine, is six.

When we do our eight minus two, we know that that is six.

So 85 minus 29 , equals 66.

Did you find the Mystery Math Mistake?

Did T-Pops do it the correct way even though he was upside down?

Let's check out to see our friends Landon and Myles to see what they discovered about how T-Pops solved the problem.

Landon said, "When I added 66 plus 29, "it doesn't equal 85, something isn't correct here."

Landon is wanting us to do the inverse operation to see if the answer is correct.

If we add 66 to 29 to check our subtraction problem, it should equal 85.

Let's try it, 66 plus 29.

If I added my tens first, my 60 plus 20 gives me 80, and my six plus nine gives me 15.

When I add 80 plus 15, it equals 95.

Landon is correct, that isn't correct.

It should have been 85.

I really think that we're onto something here.

Let's see what Myles has to say.

Myles says, "I think he regrouped the numbers correctly "and subtracted the ones correctly.

"However, in the tens you did 80 minus 20, "instead of 70 minus 20.

"The answer would be 56."

Did you see the same mistake that Myles did?

I don't know about you, but I think we need to look at T-Pops' Place Value Mat, to help us make sense of where that error was.

On the place value mat, you can see that I have the 85 like we originally started with.

Down here I have the subtrahend 29.

If I decompose those, we can see that we need to take 20 away and nine.

We started off first looking at our ones.

Can we take nine away from five?

Let's check and see.

One, two, three, four, five.

It's not possible to take nine away from five.

So we have to rename or regroup the numbers.

So, I'm going to take this 10 and we're going to rename it into 10 ones.

Now we can see the renaming that we did on the original problem is accurate.

Seven tens, and we have 15 ones cause I have the five here.

Now, he said I did it correctly when I did the 15 minus nine 'cause I had five, six, seven, eight, nine.

And when I take that away, I only have six ones left.

Ah, I see where the Mystery Math Mistake is.

We have 70, not 80.

We only have seven tens, and we have to take away two tens or 20.

So when we have 70 minus 20, we take it away and it does not equal 60, it equals 50.

That was some really great thinking second grade Math Mights.

To apply that subtraction strategy that we've been working so hard in second grade to get, it's very important to think about the T-Pops' Place Value Mat, and think about the value of the numbers as you're renaming numbers or regrouping them to help you in subtraction.

Now let's check out our I Can Statement for the Day.

Our I Can Statement says, I can solve addition and subtraction problems in the context of money.

Take a look at these coins.

How many coins do you see?

How do you see them?

I see different coins in kind of each section of that picture that are the same.

Let's see what our friend Landon and Myles think.

Landon says, "I saw eight pennies.

"I saw a group of five pennies "and a group of three pennies."

If we look here, Landon's seeing the group of eight pennies from seeing the five and the three.

Let's see what Myles thinks.

Myles says, "I saw eight nickels.

"I saw two groups of four nickels and I know, "four plus four equals eight."

If I look here, I also see the nickels grouped in a set of four, and another set of four, to show there's eight nickels.

Are you seeing the groups of coins?

Let's look at the dimes.

When I look at the dimes, I see a group of six dimes and a group of two dimes.

All of these sets have eight coins.

Does that mean they all have the same value?

I see eight coins, eight coins and eight coins.

What do you think?

Do you think this means that all the sets of coins equal the same amount?

I don't know, let's check it out.

If we have eight pennies, we know that that equals 8 cents.

If we look at our nickels, and we do our nickels fast we can skip count by tens 10, 20, 30, 40.

These equal 40 cents.

If we do our dimes, I can use the group of six tens to get 60, 70, 80.

80 cents.

Remember, even when you see the quantity of the coin is the same, it doesn't mean the value is the same if the coins are different.

Let's check this out.

We're gonna go shopping for school supplies.

We have lots of items that we can buy.

We can buy a pack of pencils for 75 cents, a pencil sharpener for 35 cents, an eraser for 45 cents and a pen for 18 cents.

Our friend Landon, wants to go shopping.

He has these coins in his pocket.

I wonder if he buys an eraser, how much money will he have left?

I know that an eraser costs 45 cents according to the chart.

Let's count Landon's money, and then see if he has enough to buy the eraser, and if he has any change left.

So I see two quarters, three dimes, and a nickel.

Let's start first by counting our quarters, 25, 50.

We know that two quarters equals 50 cents.

Now we need to stop, and skip count by 10.

So 50, 60, 70, 80, when we switch we have to skip count by fives, 85.

We know that he has 85 cents.

If he wants to buy the eraser for 45 cents, let's see how much money he'll have left.

So if we took off 25 and we continue with the dime that would be 35, and then 45.

How much money does he have left?

25, 35, 40.

We know that he has 40 cents left.

We took the 85, and subtracted the 45 cents to get 40 cents.

Great job second grade Math Mights, taking the amount of money that Landon had, counting it, and then subtracting from what he wanted to buy to see how much money he had left.

Let's check out to see what our friend Myles wants to buy.

Myles wants to buy a pack of pencils.

Here is the coins that he has.

How much money will he have left?

We know that a pack of pencils is going to cost us 75 cents.

So I'm gonna put that up here at the top.

If we look at these coins here, we wanna be able to count how many he has all together.

Let's start off with our largest value coin here, the quarter, and then we'll go to the dimes, next to the nickels, and then the pennies.

25, we need to stop and skip count by tens 25, 35, 45, 55, 65.

Now we need to stop and count by fives.

70, 75, 76, 77, 78, 79, 80.

So we have 80 cents, and we're subtracting 75 cents.

If we took 75 from 80, we would only have 5 cents left.

Let's try that on here by getting rid of the 75 cents.

25, 35, 45, 55, 65, 70, 75.

Are we left with just 5 cents?

You could even count up from 75 to 80 to know that our total left would be 5 cents.

Great work second grade Math Mights, using your knowledge of subtraction, counting up, you were adding, you were taking away, used all those pieces to help us figure out how much money Myles had left if he was going to buy the pack of pencils with the amount of money he had.

When did you use addition in the problems?

And when did you use subtraction?

Our friend Myles says, "To figure out "how much money there was, I had to add.

"Then, I subtracted to see how much money was left."

Did you realize you were actually doing a multi-step problem by adding the total of the coins, and then subtracting to get our answer?

Great work!

This time, I want you to imagine that you're going to the store to shop with a dollar.

You could buy a notebook for 26 cents, colored pencils for 18 cents, a pencil box for 39 cents, or a glue stick for 44 cents.

Our friend Clare wants to buy a pencil box, and colored pencils.

How much money will it cost?

Clare has a dollar, how much money will she have left?

Let's see how we should solve that.

Now we need to look to see how much the colored pencil and the pencil box costs, add it together, and then we can subtract it from her dollar.

We have 39 cents for our pencil box, and 18 cents for the colored pencils.

If we add this together, nine cents plus eight cents we know is 17 cents.

If I bring my one ten up here, one ten, plus three tens, plus one ten, we know equals 57.

So the total that we're working with, is 57 cents.

Now we have to take that amount, and subtract it from a dollar.

Oh no, this does not sound like fun.

I think we need to call on one of my Math Might friends, Springling.

Springling is a Math Might character with fancy eyelashes, and fluffy furs.

She was born with a coily tail, because she likes to count up or back to subtract.

I think we could use her strategy here to help us.

We know that we have 100 cents or like Clare has the $1.

We know that it costs 57 cents for Clare to buy the pencil box and the colored pencils.

So we need to count up to find out what the change is.

57 cents is really close to 60 cents.

We're gonna have Springling hop, Springling hop.

How far, or how many cents is it between 57 and 60?

We know that it's 3 cents.

We could have if we wanted to, all the way from 60 to 100.

Hop Springling, hop.

We know that that is 40 cents.

If we add together our 40 plus our three, we know that she will have 43 cents left.

Great work using our strategy of Springling to help us subtract with money.

Now it's your turn to go shopping at The Toy Store.

You're gonna do exactly what we did in the show today, by being able to have a dollar and to be able to go to our pretend Toy Store, and buy different items and see how much money you have left.

Thanks for hanging out with me, second grade Math Mights.

I've had such a fun time today from our problem that we did with T-Pops with subtraction, to having all the fun that we did with money, with adding together, and taking away with a dollar and subtracting.

I sure hope that you join us for another Math Might Show soon.

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(upbeat music) Changing the way you think about Math.

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