# Lesson video

In progress...

Hi, everyone.

I'm Miss Brinkworth.

I'm going to be going through this math lesson with you today.

Let's have a look at this learning objective together before we get started.

So we are adding or, adding and/or subtracting a multiple of 100 to/from a three-digit number.

So what we're doing is we're taking a multiple of 100 and then we're adding, we are practising adding or subtracting that from those three-digit numbers.

And we're going to be focusing on strategies that allow us to do this mentally, which means we shouldn't have to write down any jottings or any equations.

So our lesson agenda for today is that we're going to look at how bar models and part/whole models can allow us to work out what a question is asking us.

We're going to think about how partitioning can help us add those multiples of 100 and how partitioning will also help us subtract those multiples of 100.

And then there'll be that independent work.

That will give you that opportunity to practise what you've learnt.

And then the quiz at the end will give us an opportunity to see how well that learning's gone in.

Okay.

So for today's lesson, you will need a pen or pencil and some paper, and it would be really useful if you could find some online Dienes.

So pause the video here and see if you can find that equipment.

Well done.

Hopefully you've all been able to find that.

If you haven't been able to locate online Dienes, please don't worry.

It won't be completely necessary for this lesson.

Okay.

Here's a warm-up then, and you just need to have a go at adding or subtracting these numbers.

So I'm going to put that up.

Please pause the video and take as long as you need.

How did you get on? Now, I'm going to put the answers up, but before I do, I just want us to think about whether those questions were hard or not.

Did they take you a long time? Were they tricky? Were some easier than others? the first question, 200 add 300, was one that you could do really nicely and quickly.

So I'm sure you were all able to get that correct answer of 500.

But what process did you go through to do it? Well, I imagine what you did is do two add three is five, and then 200 add 300 is 500.

So what you've done is you've used a known fact, two add three is five, and you've also partitioned that number so you're just adding together the hundreds.

So you're doing quite a lot when you're mentally adding these numbers together.

I wonder if the adding ones were slightly quicker than the subtraction ones.

Normally people do seem to find addition a little bit quicker than subtraction.

But it's a really similar thing that we're doing here.

So seven take away four is three, so 700 take away 400 is 300.

These questions here then, when you've got the gap, the unknown, before the equals, it needs you to work out what it's asking.

So what number is it which is 500 add 400? We've just got the gap, we've just got that in the opposite way.

Okay.

So for that question, we need to subtract.

What is it that's 700, what is it that 200 needs to get to 700? To be the same as 700, it needs 500.

So five add two is seven.

Okay.

A similar question here, but a takeaway, where we've got the gap at the start of the question.

So what is 800 take away 200? Well, eight take away two is six, so it's 600, which is the same as, it's equal to, 800 take away 200.

Well done if you're getting these questions right where the gap is at the beginning of the equation, 'cause you're probably less used to seeing them like that.

Okay.

600 is 900 take away what? What have I got to take away from 900 to get me to 600? Well, I've got to take away 300.

We've got 200 here for 200 add 400 is 600.

And finally, for this question, you could have 500 add 200.

As long as they add up to 700, you've done really well.

Okay.

So let's move on to today's learning.

So here's a word problem, and we're going to work out together what it's asking us.

So it says, the Edinburgh Castle souvenir shop sold all 224 fridge magnets in one week.

They ordered 200 more and these were all sold over the weekend.

So how many fridge magnets were sold altogether? Well, what is this question asking us? We could draw a bar model to help us.

We've got that they sold 224, then they sold 200.

How much did they sell altogether, how many did they sell altogether? So we've got what we know at the top of our bar model there, and then we've got to work out what the total is.

Well, what word is it that gives us the clue that this is an adding question, do you think? Well, right at the end we've got that word altogether, which often tells us that we need to add things all together.

So let's have a work out how we would do this.

Here's our bar model again.

And remember as well that a part/whole model can be really useful.

Those are our parts, 224 and 200.

We need to add them together to get our unknown, to get our whole and answer that question.

So there's the question written out as a sum, 224 add 200.

How do we do it in our heads without writing anything down? What do we need to do? Well, we can see that our multiple of 100 here is 200.

So our learning objective, remember, was to add multiples of 100 to three-digit numbers.

So there's our multiple of 100, 200.

And as it's a multiple of 100, it's got zero in the ones and zero in the tens.

It's only got a value in the hundreds.

So we can just focus in on the hundreds column in this lesson and think about adapting the hundreds column.

So for this question, we can do two add two, because we've got 200 add 200.

So we can do two add two is four, so 200 add 200 is 400.

But be careful, because the answer isn't 400.

Yes, we've added together our hundreds, but we've still got the two in the tens and the four in the ones from that original number.

Just before I give you a chance to go on and have a go at doing one of these yourself, let me just explain once more what I've done here.

So to work out the question 224 add 200, I've identified that, as I'm adding a multiple of 100, the only column that needs to change in this lesson is my hundreds column.

I can add the two amounts in the hundreds column.

So out of those two numbers, I've got two and two.

Two add two is four, so I know that my hundreds column is going to change to four to give me that answer, 424.

Okay.

How did you get on? What known fact did you use to help you answer this question? Well, hopefully you could see that you need to add together your hundreds, 500 and 300, to give you 800, and then that tens and the ones also needs to move over to give you the correct answer, 865.

Okay.

So let's move on to another question then.

A souvenir shop had 424 bags of rock.

200 of these were mint-flavored.

The rest were fruit-flavored.

How many bags of fruit-flavored rock were there? Well, that part/whole model that you can see on the page there should give you a clue at what kind of question this is.

We've got the whole already, and we've got one of the parts.

We just need to work out what the other part is.

So it's a bit like the questions we did at the beginning in the warm-up.

What is it that I need to add to 200 to get me to 424? Or another way of looking at exactly the same question is what is 424 take away 200? What's my other part? And here's how I'm going to work it out.

So again, what I need to do is partition my number and think carefully about my hundreds.

I've got 400 in my original number.

We started with 424.

We then subtracted, took away, 200.

So four take away two is two.

400 take away 200 is 200.

So the hundreds column has changed from four to two.

424 has changed to 224 in this subtraction question.

Okay.

Remember that this is a subtraction question.

We will expect the answer to get smaller this time.

Pause the video here and have a go.

Well done, everybody.

Really good work for trying.

This is completely new learning for a lot of you, so please don't worry if you are making some mistakes here.

That's what happens when we're learning new things.

So if we focus in on our hundreds column for these two questions, we've got 500 and we've got 300.

So we need to do five take away three.

What's five take away three? It's two, and that's a known fact that you should be able to use to help you answer this question.

So my hundreds column in my original number is going to change from 500 to 200.

565 moves to 265.

Well done if you could see that, everybody.

Right.

It's the time of the lesson to pause the video and have a go at your independent task.

Take as long as you need, and come back together for the answers when you're ready.

Okay, let's see how you did.

I'm really hoping that you got some right answers, but please don't be too hard on yourself if you did make some mistakes.

Just have a look at those mistakes that you made, and can you see where you went wrong? So the first one here is a set of questions requiring you to add or subtract multiples of 100, so really asking you to apply that learning that we've hopefully embedded during this lesson.

So the first question is 456 take away 200.

So it's the 400 that we need to focus on and the 200.

This is an adding question, so I expect my answer to be greater than what I started with.

656.

The next question is a subtraction question, 843 take away 500.

Eight take away five is three, so I'm going to end up with 343.

The next question is something take away 700 gives me 214.

So I need the number which is 214 bigger than 700.

So I need to add 200 and 700 to give me 900.

Oh, it should say 914, not 912.

My mistake there.

Should say 914.

Okay.

Next question, what is it at the start there which is going to be the same as 761 take away 300? Well, hopefully you could see that what we need, it's 461 is the same as 761 take away 300.

Okay.

What do I need to add to 400 to get to 632? Sorry, 639, my apologies.

So we need to increase the hundreds column by two, and then we need to add the 39.

So I've got 239.

If you're getting these answers right, really, really well done.

These aren't that easy, and I really appreciate you sticking with it.

Okay.

What is 372 equal to something take away 300? It's equal to 672 take away 300.

Something take away 300 equals 156.

It's 456.

And then we've got 456 is equal to something add 200.

It's equal to 256 add 200.

And that last question there, a straightforward what is this add this, 555 add 400.

It's our hundreds column, remember, that we're focusing on on today's lesson.

Really, really well done.

I'll just leave that there for a moment for you to have a look at the answers.

Okay.

Let's go through the second part together.

Which of these do you think might be the odd one out? Which one behaves a little bit differently? Well done if you could see that it's this one.

And why is this one the odd one out? Well, it bridges 1000.

So it's not just the hundreds column that needs to adapt, and that's because 400 add 700 is 1100.

So rather than just the hundreds column changing, it's actually moved over into the thousands column, as it adds up to more than 1000.

So there's the answer for that one, and that's the odd one out.

And really, really well done if you challenged yourself to make up an addition and a subtraction question where the answer is 456.

Obviously there are loads and loads of answers to this, but here's a few I came up with, a few kind of quite easy ones, to be honest.

I'm sure you challenged yourself and came up with some harder answers there.

Well done, everybody.

You've worked really hard on some tricky new learning today.