(upbeat music) - [All] Math Mights.

- Welcome back 3rd graders.

My name is Mrs. Ignani and I'm here for another exciting episode of Math Mights.

Let's go ahead and check out our plan for today.

First off, we're gonna get our brains ready with a mystery math mistake.

Then we're gonna spend some time rounding to the nearest 10 or 100.

Are you ready to get your brains ready?

Let's check out our mystery math mistake.

Oh no, boys and girls, what has happened to our characters from Mathville?

It looks like they got stuck in a cyclone and their strategies are all mixed up.

Is that T-Pops holding DC's mallet?

You bet it is and here is how it works.

For our mystery math mistake, we're gonna see one of our characters share with us a problem that they are having a difficult time solving.

You and I have to be like little detectives and we have to look closely at that problem to see where their mistake went, so that we can help them solve it correctly.

Are you ready to do some detective work?

Let's see which one of our Mathville characters needs help today.

Looks like our friend Springling is all upside down and needs our help.

Springling is trying to solve 608 minus 492 and got 115.

Let's go ahead and solve this exactly how Springling did.

Looking at our equation, 608 minus 492 equals 115, Springling told me that she drew this out on an open number line, putting her subtrahend up here, 492, and her minuend back here, 608.

She then found a friendly number to jump to, and her first friendly number that she came to was 500.

So she's gonna go ahead and she marked her number line 500, and then hopped there for a distance of seven.

From 500, Springling was able to actually jump to another 100, the next closest one, 600.

She plotted it on her number line and then took a huge hop, covering a distance of 100.

From there, she was able to take a quick jump of eight spots.

Now we have to add up her distance that she hopped for a total of 115.

What do you think boys and girls?

Did you see any mistakes in Springling's math?

Let's see what our friends have to say about it.

Fariah said, I don't think Springling has the correct answer.

I know because when I add 115 plus 492, it equals 607, so it can't be correct.

If I understand what Fariah is saying correctly, she actually used the inverse operation to check to see if Springling covered the correct distance.

Looking at the inverse operation, we're gonna go ahead and add these two numbers and by doing so, we should get this number if the addition is correct.

So if I stack my problem, 492 plus 115, now I just need to add that like I would any other problem to see if it matches that third number.

Adding my ones, I have five plus two is seven, my tens, nine tens plus one 10 actually equals 100.

So I'm gonna go ahead and show my zero of my tens, and put my one down here to represent my 100 and zero tens.

Adding my hundreds, I have 400 plus 100 plus 100 equals 607.

You know what?

Fariah is correct, that does not equal 608.

Something must be wrong.

Let's see what our other friends says.

Dawson said, I think when Springling counted up, from 492 to 500, she miscounted.

It should have been eight not seven.

So the answer would be 116.

Looking it back at Springling's original problem, I see what Dawson is saying.

It actually should have been a distance of eight not seven.

So we're gonna go ahead and cross that out.

Now I know that 100 plus 16 is actually gonna give us 116, showing us that 608 minus 492 equals 116.

Excellent work boys and girls.

I know that Springling would be so very proud of us for helping her solve this mystery math mistake.

And remember, when you're using that open number line, it's very important that you calculate your distance correctly.

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

Our I can statement of the day is, I can round to the nearest multiple of 10 or 100.

Today, we're gonna have some fun using place value strips to round to the nearest 10 or 100.

I like using those place value strips because you are going to be able to really visualize what you are doing to round to that nearest 10 or 100.

Take a look at this number.

If we have the number 825, and we we wanna round this number to the nearest 10, what is our new number going to be?

In order to figure this out boys and girls, we are gonna go through some specific steps slowly so that we can do this together.

Step one, we're gonna pull away any strips before the tens place and set aside.

So if I have my 825, I'm gonna go ahead and I'm gonna pull out my hundreds because we are focusing on our 25.

We're rounding to our nearest 10, so let's check out our second step.

Step two, we have to decide what two tens places are we considering to rounding to?

Looking at our 25, we have to decide if we're going to round to 30, or if we're gonna round to 20?

Step three, to check out which way we should round, we're gonna use the abacus to explain our thinking.

So if I have my abacus and I know that it's cleared because they're all at zero, I'm gonna go ahead and show my 25.

Now, I need to decide if I want to move these five over to round to 20, or I can move these five over and round to 30.

But wait a second boys and girls, what's the deciding factor in that?

We're moving five either way?

With dealing with rounding, there's actually a tip at the bottom of our slide that says round up at five, you are at the top.

That's a rule that we have in mathematics boys and girls, when it comes to rounding.

If we're at that five, we're actually just gonna round up to that higher 10 or 100 place value.

So in this case, when I have 25 because I'm at that five, I'm gonna go ahead and round up to 30, which takes us to step four, we're gonna take the 30 and put it back with the 800.

So we're gonna go ahead and take our 30 and we're gonna go ahead and round 830.

Now we can't forget our one spot.

We have 830, so 825 rounded up to 830.

Boys and girls, wasn't that so much fun using those place value strips to round to the nearest 10?

Did you see how you were able to really visualize it when we use those strips?

Let's go ahead now and see if we can round to the nearest hundred.

We have a new number, 428.

We wanna round this number to the nearest 100.

It's asking what our new number is.

So let's first go through these steps to create it.

So, first thing I need to do is to consider which hundred I'm going to round to?

So if I have 428, I have to decide if I'm rounding to 400 or if I'm rounding to 500.

Next, I need to plot this out on a number line to help me make that decision.

So if I'm gonna write out my number line and I'm gonna plot my 400, my 500, I always like to put a midpoint in there to help me gauge which part of the number line it falls on, so my midpoint 400 and 500 is 450.

So middle of the way, 450.

Then seeing that my number 428 is actually gonna fall over here.

Now with it falling below 450, I know that I'm actually going to round it to 400.

So now that I have decided that I'm gonna round to 400, I need to go ahead and finish to represent that.

I'm gonna go ahead and get rid of my 500, and I now have my 400 and I have my zero tens.

And now my zero ones to show that I have rounded 428 to 400.

Excellent job using those place value strips to round both ways.

Let's see now, if we can do a little bit more practice, either rounding to the nearest 10 or the nearest 100.

So, we have five numbers and we're gonna see what it is when we round each number to the nearest 10, and then when we round that same number to the nearest multiple of 100.

If I have the number 72, I need to decide does it make sense to round 72 to 70, or does it make sense to round that 72 up here to 80?

Now, thinking about what we already know about rounding, I know that it makes more sense to round 72 down to 70.

So my nearest multiple of 10 is 70.

Taking that same number 72, I need to decide, am I going to round up to my nearest 100, or with 70 to make more sense to round down to zero?

In this case, I need to picture that number line in my head, thinking about where that midway point of 50 would fall.

Thinking of 50, 50 would fall over here, making that 72 would round up to actually 100.

So rounding to the nearest multiple of 100, 72 is going to be 100.

Looking at our second number 418, we are going to try to round that to the nearest 10.

If we have 418, we have to decide if we wanna round it down to 410, or would it make sense to round it up to 420?

Now, thinking about what we know about that eight, we know we're gonna round up to 420.

So our nearest multiple of 10 is 420.

Taking that same number 418, now we need to round it to the nearest multiple of 100.

So again, we have to picture that number line in our head.

If we have 418, we need to think about where on that number line would that midpoint 450 fall?

It would fall over here.

So we know that 418 is going to round down to 400.

For our third number, we now have the biggest number we've dealt with so far, 798.

Same procedure though, first we're gonna round it to the nearest 10.

This number is a little bit tricky, 798, when we're rounding to the nearest multiple of 10.

So I'm gonna go ahead and actually plot this on a number line, so that we can make sense of it a little bit more clearly.

If I have my number line and I'm deciding in my tens, if I want to round to 90 or 100, which is the next closest 10, I'm gonna plot that 98, and it makes sense that we actually would round up to make it 100.

And so, what that's gonna do boys and girls, is that's not only gonna change our tens column, but also our hundreds column too, because we're renaming and regrouping it based on those laws of rounding.

So our 798, we're gonna round that 10 up and it's actually gonna become 800.

So 798 rounding to the nearest multiple of 10 is 800.

So what's nice now boys and girls, is now that we've actually seen that 798, rounding to the nearest 10 is actually 800, when we're now doing our second step and thinking about rounding 798 to the nearest 100, well, we already know it's 800.

So I'm gonna go ahead and fill in our chart because 798 rounded to the nearest multiple of 100 is 800.

Our next number boys and girls, 349 again, we're gonna look at that tens to round to the nearest multiple of 10 first.

If I have 349, I need to decide am I going to round up to 350, or am I going to round down to 340?

Looking at my one's column, I know that 349 is closer to 350, so when I'm rounding that number to the nearest multiple of ten, 349 rounds up to 350.

Now that we have rounded to our nearest multiple of 10, we're gonna take 349 and round it to the nearest multiple of 100.

Again, I have my 349 and I have to picture in my mind that open number line thinking about where is that midpoint?

If I have 300 and I have 400, that midpoint is going to be 350.

Now, 350 is gonna fall right here, so I know that I'm gonna around 349 down to 300 if I'm rounding to that nearest multiple of 100.

Great job rounding today, boys and girls.

Now it's your turn to play tic tac round, and you are gonna apply everything you learned about rounding to the tens or the 100s with a friend.

Great job today boys and girls, we started off our show, helping our good friend, Springling work through that mystery math mistake and came to that correct answer for her.

Then we had a lot of fun exploring how we can round in multiples of 10 or 100.

I had such a blast with you today, I can't wait for the next time we're together working on our math skills, but until then, bye.

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

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