Lesson video

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

Today, we'll be looking at solving an addition and subtraction word problems. Let's have a look at today's lesson agenda.

First, we'll look at representing word problems. Then we'll have a little bit of let's explore.

Then we'll be solving word problems. And finally, you'll be off your independent task.

So for this lesson, you will need a pencil and notebook.

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

Super.

Let's get started.

So here are builders, hard at work.

In the morning, the builder started with 46 bricks.

They used 14 bricks.

How many bricks did they have left? Hmm.

Well, what is this question really asking me? First of all, what do I know? I know the builders started with 46 bricks and they used 14.

And now I, what I don't know is how many did they have left.

To help me represent this, I'll use a part-whole model.

Hmm, do I know my whole? Actually I do.

The builders started with 46 bricks, so I know that's my whole, and they used 14 of them, so that must be one of my parts.

Because the other part I don't know, I want to know how many did they have left after they used 14.

So if I know my whole, and I know my, one of my parts, and I need to find another part, what calculation will I need to, will I need here? Hmm.

Well, I'm know I'm going to use subtraction because I'm going to subtract my part that I know from my whole.

46 minus 14 will give me my other part.

Let's take a look at another equation.

So here again, the builders are really hard at work.

The builders are building two walls.

One wall is 24 bricks long, the other wall is 8 bricks long.

What is the total length of the walls? Hmm.

Again, I think I'm going to use a part-whole model to help me here.

So let's start with what do we know.

Well, we know one wall is 24 bricks long and the other wall is 8 bricks long.

What I don't know is the total length of the walls.

So actually, I don't know my whole, but I do have my parts.

I know one part is 24 and my other part is 8.

So if I have two parts and I want to know my whole, what calculation will I need here? Have a think.

What operation do we need? Oh, I know what we need to do.

We need to add them to get our whole.

24 plus 8 should equal our whole.

So let's explore some more problems. What I'd like for you to do is I like for you to match the word problems to their whole part models.

And as usual, I'll do the first one before you go off on your own.

So in the first one, it looks like Jessica walks 42 steps and Henry walks 16 steps.

How many steps did they both take? What do I know here? Well, I know Jessica walks 42 steps and Henry walks 16 steps, but I don't know how much they both take together.

I don't know my whole, I only know my parts.

Hmm.

I know, 42.

Oh, here we go.

In C, it looks like 42 and 16 are both parts, but we don't have a whole.

So it looks like the C, part-whole model C matches my first word problem.

Your turn, match the word problems to the part-whole model, and when you're ready, resume the video and we can go over the answers together.

So pause now.

Good luck.

Great job, everyone.

Let's go over these answers together.

We already did Jessica and Henry, so let's move on to Sandra.

Well, Sandra had 40 coloured pencils and she gives 20 coloured pencils away, how many are left? Well, I know that she has 40 coloured pencils, that's how much she started with, that's my whole.

She gave 20 away, how many are left? So it looks like I have a whole, a apart, and I need another part.

Well, that matches A.

40 being the whole 20 being the part.

William has a book which is 42 pages long.

He reads 16 pages.

How many pages does he still have to read? Well, I know the whole book is 42 pages long, so that's my whole, I know my whole.

He's read 16, which is a part, but I don't know the other part which is how many he still has to read.

So that matches B.

A whole of 42 pages, him reading a part, which is 16, and I don't know the other part, how many pages he has left.

And it looks like a can of juice costs 40p and the sweets cost 20p, how much do they both cost? Well, I know that juice is 40 and the sweets are 20, but together they both cost, well, I don't know, I don't know my whole.

So that would match D.

Two parts, 40 and 20, missing the hole.

Let's move on.

So we're back to our word problems, and our part-whole models that we were looking at before, before our let's explore.

Now, what I'd like for us to do is use our strategies to think of how we can solve this.

So I know that my equation here is going to be 46 minus 14, because again, I know the builders start.

Oops.

I almost gave you the answer.

I know that the builders started with 46 bricks and they used 14.

So now I need to know how many they have left.

46 minus 14, what strategy do you think is best? How would you solve this? If you know how to solve this, pause the video now and resume when you're ready.

But if you want to stay on with me, of course, that's fine.

So my favourite strategy when subtracting two digit numbers is to partition the second number.

So I'm going to partition 14 into 1 ten and 4 ones.

So let me think, 46 minus 10 is 36, 36 minus 4, well, 6 minus 4 is 2, so 36 minus 4, oh, that must be 32.

So the missing part is 32.

Well done, everyone.

Now, I want to make sure that my mental calculations were correct.

So what I'm going to do is the inverse for the opposite to double check my answers.

So my inverse is 32 plus 14 should be equal to 46.

So 32 plus 14, again, I'm partitioning 14 into 1 ten and 4 ones, 32 plus 10 is 42, and then 42 plus 4, well, I know 2 plus 4 is equal to 6, so 42 plus 4 is equal to 46.

Excellent.

I got that right.

So shall we try one more together? Super.

So again, we have our builders and as we said before, they're building two walls.

One wall is 24 bricks, and other wall's 8 bricks long.

So I have two parts there, and I need to know the total length of the wall, I need to know that whole.

So I'm going to add these two numbers together.

So what I like to do.

Actually, well, let me ask you, how do you think we should add these numbers? What strategy would you use? Hmm.

Well, when I add, and I know I'm going to cross a 10, I like to partition the numbers so that I can use to make 10 strategy.

So I have 24, I know 4 plus 6 is equal to 10, so 24 plus 6 will be equal to 30.

So I'm going to partition that 8 into 6 and 2.

So 24 plus 6 equals 30, 30 plus 2 is equal to 32.

Super.

So what I need do now is use the inverse to check my answer.

So the inverse of addition, the opposite of addition is subtraction.

Okay, so we're going to go backwards and hopefully 32 minus 8 will be equal to 24.

So again, let's think.

I know 32 minus two is equal to 30, so I'll partition the 8 into 2 and 6.

32 minus 2 is equal to 30, 30 minus 6, well, I know 10 minus 6 is equal to 4, so 30 minus 6 must be equal to 24.

Yay.

I got that one right.

And if you went off on your own and did it, wow, super job.

So, oh my goodness, it is already time for your independent task.

That was a really fun lesson so it went by really quickly.

What I'd like for you to do for this independent task is to solve the following word problems by creating a whole part model and using the strategy you think is best to solve them.

So remember our whole part models, if you're still unsure, you can always go back and take a look at the models that we made earlier in the lesson.

So as usual, I will complete the first one, and then you can go off on your own so you're really clear on what to do.

So let's look in the first one.

Bob the builder's house has 16 windows.

His workshop has 32 windows.

How many windows are there altogether? So I know Bob's house has 16 and his workshop has 32, and we need to know how many there are altogether.

Oh, so what do I know? I know my parts, 16 and 32, but I don't know my whole, so I have to use my parts to find my whole, which means, I need to add 16 and 32.

So again, the strategy I really like is partitioning the second number.

32 plus 16, 16 can be partitioned into 1 ten and 6 ones.

32 plus 10 is equal to 42, 42 plus 6, well, I know 2 plus 6 is equal to 8, so 42 plus 6 is equal to 48.

And if you're looking for an extra challenge, you can do the inverse to check your answers.

Again, your turn, pause the video, when you're ready, you can resume and check your answers with me.

Good luck.

Great job, everyone.

So let's take a look.

Looks like Naomi had 38 pence and she used 17 pence to buy some sweets, so she has 21 pence left.

You should've subtracted for that one 'cause you knew the whole and one part.

Sally made toys, she made 65 in total and sold 32.

Wow.

Those are some cool toys.

So she started with 65, that was my whole, we had to subtract 32, that means she has 33 toys left.

Ooh, James and he's cake baking.

He added 70 grammes of flour then he added 25 more, how much flour is there now? Well, 70 plus 25 are my parts, and together, the whole is 95, so 70 grammes plus 25 grammes is equal to 95 grammes.

Super job, everyone.

If you'd like to, you can ask your parent or carer to share your work on Instagram, Facebook or Twitter, tagging @OakNational and #LearnwithOak.

As usual, don't forget your final quiz.

I really look forward to seeing you in future lessons.