Lesson video

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Hello, everyone, I'm Miss Brinkworth.

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

What we're going to be doing is subtracting one three-digit number from another.

We're not going to be using any regrouping during this lesson.

So we're going to be focusing on mental strategies to help us.

So make sure that you've got somewhere lovely and quiet to sit and we'll get started.

Hello, everyone.

So like I said, in today's lesson, what we're going to be doing is taking one three-digit number away, subtracting it from another three-digit number.

This isn't going to require any regrouping.

So what we mean by that is that in each column, the number that we're subtracting is smaller than the number we're taking it away from, so we don't have to do any regrouping.

And hopefully, once we've got to the end of the lesson, you'll be able to use your strategies to do these sums mentally, which means doing them in your head, not having to write anything down.

So if we look at today's lesson agenda, what we need to do for today's lesson, I'll just get rid of my webcam, is we're going to recap the mental subtraction strategies.

I'm sure there's strategies you already feel really confident on when it comes to mental subtraction.

Then we're going to look at how partitioning larger numbers can help us to subtract.

Moving on to doing that mentally.

And then you'll have time for some independent work, where you will have a chance to practise your new skills.

And then an exit quiz where you can see how well you've got on with today's learning.

So please make sure you've got a pen or pencil, some paper.

And it'd be really useful if you could find some online Dienes.

So Dienes are the little different-sized cubes that we use in class to help us when we're talking about place value.

So if you could ask a parent or carer to help you find some of these online, and help you use them throughout the lesson.

Pause the video here and find what you need.

Okay, once you found what you need, here's a warmup for you.

So pause the video again, and think about which calculation helps you with each of the subtraction questions there.

So a line is going to appear, you need to match them up.

Which known fact on the left helps you answer the questions on the right? And then if you've done that, a little challenge is can you identify any other number facts that you've used? Okay, so here is how they match up.

Five take away one helps you with this larger number of 52 take away 11 'cause we can do 50 take away 10.

And then this one helps you here.

Nine takeaway three equals six can help you with 98 take away 61.

And then seven takeaway two can help you with this one, 70 take away 20.

And other number facts that you might have used.

Well done if you identified some of these ones as well.

Really, really good, okay.

So here's the type of question that we're going to be looking at today.

On Friday, the tour guide showed the Tower of London to 339 visitors.

On Saturday, she showed it to 123 fewer visitors than she did on Friday.

How many visitors did she show the tower to on Saturday? So the first thing we need to think about is what type of question this is.

So what can help us here is identifying the numbers.

So we know that on Friday she showed around 339, and on Saturday she showed around 123 less.

So what that might look like if we get a bar model up is something like this.

So we've got our knowns and our unknowns in this question.

We know that on Saturday, she showed the Tower of London to a larger group of people.

And then she showed it to 123 less on Friday.

So that missing information there is our unknown.

And hopefully, you can see with it set out in its bar model like this that this is a subtraction question.

We need to take 123 away from 339 to find the answer to this question.

Okay, so there's our sum.

How do we do it then? There's our bar model again, which just shows us those knowns and unknowns, and what we need to find out.

We've got the whole, which was the amount of people she showed it around on Friday.

We've got the amount that we know were shown less, and we need to find that other part to answer this question.

So what you might want to do is think about using those online Dienes.

So now would be a really good time if you do have those online Dienes to get them open, and think about creating these numbers using them.

So here, I've got my three hundreds for 300, my three tens for 30 and my one nines for nine.

And what I need to take away from that is one hundred, two tens because it's 20 and three ones.

What will it look like when I've taken them away? Well, I've got one less hundred.

I've got two less tens and I've got three less ones.

Can you count up my Dienes that I've got there, and see what my final answer is? What have I got there on the right? Well, hopefully you can see, I've got two hundreds, one ten and six ones, which means my answer is 216.

Okay, here's another one then.

And here, as you can see, I've got the part-whole model to help you.

We've got 339 take away 24.

So 339 is our whole and we've got one of our parts.

We need to work out what the other part is.

What is the other part to make up 339? That's just a different way of thinking about exactly the same question.

339 take away 124.

So let's have a think then about getting those Dienes again.

So they look a little bit different here.

They're different colours, slightly different sizes, but they represent exactly the same thing.

So you can see I've got three hundreds, three tens and nine ones.

One hundred, two tens and four ones.

Get those together and have a think about what my answer's going to be.

Do you know what's going to appear in my hundreds now? So I've got three hundreds and I'm going to take one away, so I'm going to end up with two hundreds.

I've got three tens there at the beginning, and I'm going to take two of those tens away.

When I've done that, I'm only going to have one ten left.

And then the same for my ones.

I've got nine ones.

I'm going to take all ones away, so I'm going to end up with five ones.

How much have I got there at the end then? I've got two hundreds, one ten and five ones.

I've got 215.

Now, our Dienes are really useful to lay out how to do this.

But actually, if you look at this question, hopefully you can see that your Dienes aren't completely necessary.

If you think about what we're doing with our hundreds, if we just focus on our hundreds for the moment, we had three, we took away one, we've got two.

The sum that we did there was three take away one is two.

You all know really confidently how to do sums like that.

So we don't always need our Dienes.

But for this question here, you can use them.

So ask a parent or carer to help you find those online Dienes, and have a go at this question.

So pause the video here, and have a go at answering this question.

Okay, let's see how you got on then.

So you might want to have found some Dienes, and they should have looked like this when you started, 338.

That's 338 in Dienes, what did your 124 look like? 134, sorry, you should have three tens.

And once you've taken that smaller amount away from the bigger amount, you'll have this.

So we've got no tens left because we've got the same amount in our tens column.

They cancel each other out.

We had three tens, we took three tens away.

So we haven't got any tens anymore.

We had three hundreds, we took one away, so we've got two left, two hundreds.

And we had eight ones, we took four ones away, and we've got four ones left.

So what is that there that you can see? What have I got left as my answer? Well done if you say I've got two hundreds and four ones.

And well done if you were able to make that with your Dienes as well.

Okay, so let's have a go at answering these questions without Dienes this time.

So the Dienes are a really good way to visualise them.

And you might want to carry on sort of seeing the Dienes in your head when you're thinking about taking away the hundreds, the tens and the ones.

But it's not completely necessary to use your Dienes.

So if we think about partitioning the number up, dealing with the hundreds, the tens and the ones, just like we've been doing when we've been practising with our Dienes, what we're doing is 800 take away 300.

Now, I know that eight take away three is five.

So 800 take away 300 has got to be 500.

Another nice easy one, 80 take away 10.

Well, I use my fact eight take away one gives me seven, so 80 take away 10 gives me 70.

And for my final question, I've got four in my larger number in my ones column.

I've got two in my smaller number.

So the question for my ones is four take away two, which is two.

I'm not finished though.

I need to put all of those answers together.

I've split them up.

I've partitioned them to help me answer the question, but I've got to put them back together again for my answer.

But this is simple.

I've got 500 and 70 and 2.

My answer, 572.

Okay, your turn then.

Can you have a go at answering this question not using your Dienes this time? How did you get on? Shall we have a look at it together? So hopefully you started with your hundreds.

800 take away 600 is 200.

Then with your tens, 90 take away 50.

You'll use that fact nine take away five gets you four, so 90 take away 50 gives you 40.

And four take away one is three.

And that gives you 243.

Really, really good.

Right, it's time for you to pause your video, and have a go at your independent work.

Please start the video again when you're ready.

Okay, let's go through the answers together.

And please do check your answers carefully.

And carry on listening to the video right to the end, even if you've made a few mistakes.

That's honestly absolute fine.

We all make mistakes when we're learning things.

But do have a think if you have got a question wrong maybe where you went wrong.

Have a go at the question again.

Try your working out again, and see if you can get it right next time.

So here are the questions that you had for your independent task.

I'm not going to go through them in lots of detail because there's lots of them, but I will put up the answers for you for you to have a look at.

And really, really well done if you got those answers correct, especially those ones where you've got the sort of answer at the beginning and where you've seen that it's exactly the same sort of question.

You've just got a question like the one in the middle, 409 take away 203.

It's exactly the same type of question, but we've just got that missing value at the start.

Okay, well done as well if you went on to the challenge, where it's subtract two three-digit numbers to get 222.

How many different ways could you find? I found a few different ways here, but these are all really quite simple, kind of straightforward, boring ways.

I'm sure you found much more interesting, and far more ways as well.

Okay, part B, which of these is the odd one out and why? Well, hopefully you could see that this question here was different to the ones we've looked at so far.

And that's because it needed regrouping.

So as we can see, seven is bigger than two, and three is bigger than zero, so that's a question that needs some regrouping.

We can't necessarily do that question mentally.

Here's one way of doing it.

It's not necessarily the best way, or certainly the only way of doing it, but I've shown that you can partition the number up.

So partition 802 into 102 and 700, and then take away the amounts separately to get you the right answer.

And really well done if you challenged yourself there.

Now, remember if you would like, please ask your parent or carer to share your work on Twitter, tagging @OakNational and #LearnwithOak, especially if you did this part here, where you managed to find lots of different ways of making 222.

We'd be really interested to see all the different ways you managed to do that.

And before you go, please have a go at that final knowledge quiz 'cause that's a really good way of seeing how well you've got on with today's learning.

Really good work today, everybody, well done.

Have a fantastic rest of your day with your learning.