# Lesson video

In progress...

Hello everyone, and welcome to math with Ms. Dobrowolski.

Today we'll be applying multiplication, and division of mass.

So let's have a look at today's lesson agenda.

First we'll have a look at just multiplication, and division.

Then we'll have our talk task.

And then we'll have a look at multiplication, and division of mass.

For this lesson you will need a pencil, and a notebook.

If you don't have these items pause the video now, and go get them.

Super.

So.

The mass of two parcels is 10 kilogrammes.

And each parcel has the same mass.

What is the mass of each parcel? Hmm.

How could we find the mass of each parcel? Let's draw a bar model to help us.

So what do I know? Well I know the mass of two parcels is 10 kilogrammes.

So I know my whole.

Okay I know my whole is equal to 10 kilogrammes.

So I have one two three four five six seven eight nine 10.

So I know my whole is 10 kilogrammes.

So I've put it in this bar model here.

What I need to do now is figure out how many parts I have which is two.

Because I have two parcels.

So now I need to split my parts into two equal pieces because each parcel has the same mass.

So if I have 10 kilogrammes, and I split it into two parts.

Each part will have one two three four five kilogrammes.

One two three four five kilogrammes.

So 10 kilogrammes divided by two is equal to five kilogrammes.

So 10 kilogrammes divided into two equal parts is five kilogrammes.

The mass of each parcel is five kilogrammes.

Let's try another one.

The mass of a pencil is five grammes.

What is the mass of eight identical pencils? So again let's draw a bar model to help us.

So what do I know? I know the mass of the pencils' five grammes.

But I want to know what is the mass of the eight identical pencils.

I don't know my whole.

I don't know the mass of all eight.

But I do know that each of my parts has a value of five.

And I know that I have eight parts.

Because I have eight pencils.

So here I have my eight parts that are each have a value of five grammes.

One two three four five six seven eight Yep eight parts.

And I need to know my whole.

What is the value of all of these parts together? Well.

Five part five grammes times eight parts that's equal to 40 grammes.

Five 10 15 20 25 30 35 40.

So that means eight multiplied by five grammes is equal to 40 grammes.

That is the mass of all eight pencils.

Super.

So you need to figure out Oh am I doing multiplication or division to solve this problem? So let me do the first one so that you're super clear on what you need to do.

So I can see here Mr. baggers has two parcels.

Each parcel has the same mass.

What is the mass of each parcel? Okay.

So I know that the mass of both parcels is six kilogrammes because that's what it says on the scale.

Okay.

So one two three four five six kilogrammes is my whole.

Okay? I know six kilogrammes is my whole so the total mass is six kilogrammes.

That means I need to take my six kilogrammes, and I need to make sure I have two parts.

Because I have two parcels.

So I have six kilogrammes, and I divide it by two.

Because I have two parcels or two parts.

And let's see if I divide by two, each part will have a value of one two three.

One two three.

Of three kilogrammes.

So the mass of each parcel is three kilogrammes.

Pause the video, complete the talk task, and I'll see you when you're finished.

Good luck.

Great job everyone.

So we can see in number two that we had two letters.

Each with a mass of 30 grammes.

So if I wanted to know the total mass I need to multiply 30 times two, which is equal to 60.

And then for number three, the total mass of the three pencils is 30 grammes.

So I wanted to know the mass of one of one part.

So I take my whole my 30, and divide it by three.

30 divided by three is equal to 10.

Great, let's move on.

So here I have two scales.

Three 100 gramme masses are needed to make the needle point as shown here.

So in order to make the needle point move to this spot, I need three 100 gramme masses on it.

How many 100 gramme masses are needed to make the needle point as shown here on the red scale? Assuming they have the same scale.

So I know the needle point needs 300 grammes mass to to point here.

I want it to point here.

Mhhh.

So how many how many hundred grammes masses do I need? Well, I know that to get the needle point to the first quarter, I need 300 grammes.

Okay? This is an this is my second quarter.

I can see that this scale is cut into four quarters.

One two three four.

So if the first quarter is equal to 300 grammes that must mean I need another 300 grammes to get the arrow to point to the bottom.

So 300 plus another 300 or 300 doubled is equal to 600.

Now, this is another quarter.

So that means I'm going to need another 300 for it to point here.

So 600 plus another 300.

I no six plus three is equal to nine.

So 600 plus 300 is equal to 900.

So great.

If I want the needle to point to 900 grammes, how many 100 gramme masses join me.

Well if my total is 900, and I know each one is worth 100, that must mean 900 divided by 100.

Uh that's a really big number.

But actually I can help I can use my knowledge of nine nine divided by one.

You see if each piece is worth 100 that means I need nine 100 grammes.

100 200 300 400 500 600 700 800 900.

So I need nine 100 gramme masses to make the needlepoint show as shown here.

Now, let's have a look.

One plate has a mass of 20 grammes.

What would the total mass of 10 plates be? As usual I always like to draw a bar model to help me.

So let's have a think.

How could we solve this problem using a bar model? Do we know the value of the whole? Mmmhhh No.

Coz we want to find out the total mass.

So we actually don't know the total.

We don't know the whole.

Do we know the value of the parts? Well I know that I have 10 plates, and each has a mass of 20 grammes.

So the value of each part must be 20.

And there must be 10 parts.

Because there's 10 plates.

So here are my 10 parts.

One two three four five six seven eight nine ten.

And I know each one has a value of 20 grammes.

So now I need to find my whole.

Well if I know my parts, and I know the value of each of my parts.

I need to multiply them to get the whole.

20 grammes times 10.

Which is equal to 200.

We can also double check by counting in 20s.

20 40 60 80 100 120 140 160 180 200.

So 20 grammes times 10 is equal to 200 grammes.

Now, what if I said 10 plates times 20 grammes? Would I still get the same answer if I reversed the multiplication? Well, 10 times 20 grammes is still equal to 200 grammes.

20 40 60 80 100 120 140 160 180 200.

So in multiplication it doesn't matter which number comes first.

As long as you're multiplying two of the same numbers together, you'll always get the same answer.

It doesn't matter which one comes first.

Let's try another one.

The total mass of five bricks is 20 kilogrammes.

What is the mass of one brick? Okay, so let's have a think.

How can we use a bar model to help us? So do we know our whole? Yes we do.

Because the total mass of five bricks is 20 kilogrammes.

So I know my whole is 20 kilogrammes.

Do I know how many parts I have? Mmmhh Well I know the total mass of five bricks is 20 kilogrammes.

So that must mean I have five parts.

One two three four five.

Mmmhhh Okay.

So I know my whole I know my parts.

Do I know the value of my part? Do I know how much each brick is worth is has is worth.

No.

That's what I need to figure out.

I need to know what is the mass of one brick.

So what I'm going to do here is I'm going to take my 20 kilogrammes, and divide them evenly by five bricks.

So watch me.

So I need to take my 20 kilogrammes, and divide it by five bricks evenly.

So one two Count with me.

Three four five let me go back to the beginning.

Six seven eight nine 10 11 12 13 14 15 16 17 18 19 20.

So i divided evenly between the five bricks.

So how many are in one? How many are in one brick? One two three four.

So one, the mass of one brick is equal to four kilogrammes.

So again, when I was dragging each kilogramme into the brick.

I was dividing evenly.

So 20 kilogrammes divided by five is equal to four kilogrammes.

And that's because each brick now has a mass of four kilogrammes.

One two three four one two three four one two three four one two three four, and one two three four.

This part always sneaks up on us.

What I would like for you to do is to solve the following multiplication, and division problems. You will need to draw bar models to help represent the word problems. So remember, draw a bar model because that's going to help you decide if you need to complete multiplication or division.

So pause the video resume when you're ready to go over the answers.

Good luck.

Great job everyone.

So let's see.

In number one, we should have completed a multiplication problem.

That's because I knew that each of my parts was equal to 50 grammes, and I had four parts.

One two three four.

So 50 times four is equal to 200.

50 100 150 200.

So the marker on the red scale should be pointing at 200.

Just like I have it here.

For number two, we knew the mass of one box of pens was equal to 40 grammes.

So we knew our whole.

And we knew that we had five pens in a box.

So we knew that each pen that each that the 40 grammes could be divided by five.

Because we wanted to know the value of each of our parts.

So 40 divided by five is equal to eight.

For number three, we knew that there were 10 bricks, and each brick had a mass of two grammes.

So we knew that the total we wanted to know the total mass of 10 bricks.

So we took our parts one two three four five six seven eight nine 10, and we multiplied them by two kilogrammes.

Ten times two is 20.

Or you could have skip counted by twos.

Two four six eight 10 12 14 16 18 20.

And you would have gotten 20 kilogrammes.

Really good work everyone.