Lesson video

Hello everyone.

I'm Miss Brinkworth.

I'm going to be going through the subtraction lesson with you today.

So let's have a look at our learning objective.

Our learning objective for today is to subtract three-digit numbers and this is going to require us to regroup from the hundreds into the tens column.

I know that column subtraction is absolutely everybody's favourite, and by the end of today's lesson I'm hoping you're going to feel really confident with it.

If we have a look at today's lesson agenda, we're going to recap some mental strategies first and that's going to help us as we move through our column subtraction.

Although column subtraction is a written message, we do still need those mental strategies to subtract in each column.

We're then going to have a look at dienes and how they can help us visualise what we're doing when we regroup.

It will really make it clear why we regroup when we're subtracting like this.

Then we're just going to move on to column subtraction.

While both dienes are really useful, the beginning of the lesson to give us an idea of why regrouping is necessary, I'm hoping by the end of the lesson, most of you will feel confident to answer these subtraction questions just with that column subtraction and without the dienes.

Then you'll have that opportunity for some independent work where you can practise, really perfect it, and then there's that quiz at the end where you get to see how many you can get right.

Okay, absolutely crucial for today's lesson is paper and pencil, please.

If you can, ask a parent or carer to help you find some online dienes.

They will be really useful for today's lesson, but if you can't find them, please don't worry.

Please continue with the lesson.

I will put them up on the board for you to see.

So pause it again here, and go and get everything you need.

Wonderful, let's get started.

Here's a warm up for you.

Have a look at these two questions.

There are mistakes.

They haven't got the right answer.

If you can find the right answer, really well done.

And if you can maybe try and see where they've gone wrong, even better.

Take as long as you need.

How did you get on? If we have a look at that first question together, we can see that they've made a mistake here in the tens column.

This is because they should have moved one of those tens into the ones.

They needed to regroup in the ones column because they've got a larger number there.

They're doing one take away six, so then you just take one of those tens and you put them in the ones.

That meant that when they became into the tens column, the sum they should have been doing was four take away four, not five take away four.

That's the mistake that they've made there.

In the second question, what they've done in the one is that they have decided instead of doing eight take away nine, they decided to do nine take away eight gets us one.

That's the mistake they've made there.

They needed to regroup from their tens to get that question right.

Now these are two of the most common mistakes that people make when they're doing column subtraction and when they need to regroup.

The first one is, that they are forgetting to cross out when they've regrouped from a larger column, tens or hundreds, they've forgotten to cross it out and write the new amount.

In the tens column there for that first question, they should have crossed out the five at the top, and written four to remind themselves when they came back to the tens column, that they were working with four now.

Five's gone, they should have been working with four.

For the second question, we can see what they've done is switched around the order in which they should be subtracting.

These are two issues that we will look at again in the lesson, but it's really important for you to be aware of because when people make mistakes in column subtraction, this is normally where they make them.

What can you tell me about this calculation shown here with this ? Well hopefully you can see that some regrouping is needed.

It's not really needed in the ones column, because in the one's column, the two numbers that we are talking about are five take away four.

That one's easy without regrouping.

The regrouping comes in the tens column where we've got six in the whole of 465, and we've got seven in the part 274.

This is what today's lesson is all about.

When we're having to regroup from this in the tens column.

So let's take this sum and have a look at what we can do with it.

So here I represented it with my dienes.

I've got 465 take away 274.

I've made the tens column pink there and hopefully you can see with the dienes, why this becomes a calculation that requires a little bit more work from us, why we do need to regroup.

And that's because you can see, hopefully, that there are a larger number of tens in the part, so we need to regroup.

And today we're going to be regrouping from the hundreds.

What we're going to be doing is working with the fact that a hundred is ten tens.

So we can exchange one hundred for ten tens, because it's exactly the same thing, we just move it from the hundreds column into our tens column.

Let's have a look at what that looks like.

We're working with the same sum, 465 take away 274.

What I'm going to do is go through representing each of those numbers with dienes, exchanging and regrouping, and then subtracting with the dienes as well.

I'll go through this really slowly.

I do apologise if this is something that you are quite good at.

If you already know how to do this, that's fantastic.

But it will help you visualise why we do it.

It's not going to do any harm to just have a look at what we do with the dienes.

I'm going to put it all on the board with you in dienes, but if you would like to do this with your online dienes at home as well, that would be wonderful.

I'm going to make 465, so I need four hundreds, and how many tens do I need? I need six, because I've got 65.

And then I've got my five ones.

Then I need to make 274.

Two hundred and seven tens, and four ones.

So you can see here that it's going to be tricky when I get to the tens column.

But first, I can do my ones.

I need to take away four ones.

Sorry, first I'm going to exchange my hundreds.

When you do this with column subtraction, you will always start with your ones.

But as I'm going through with you, a lot of the time I've done my exchanging, my regrouping first just to show you exactly what I'm doing.

If I just go back, what I've done first is I've got rid of one of my hundreds and I'm replacing it with those ten tens.

I'm then going to take away what I need to from the bottom row.

So four ones and then I can see that I can take away my seven tens.

There's my seven tens.

I then need to take away my two hundreds.

Sorry, let me go in the right order.

Take away my two hundreds and then what I'm left with, which is 191.

So if we look at what that looks like with column subtraction, we can just see over here why we are having to be great on this sum, because we've got on this calculations area, and that's because we've got a larger number of our tens of the parts, we've got seven, and it's a smaller number in the whole where I've got six.

Like I say, normally you would just start with the ones but I'm just going to start with the regrouping just to show you what we are doing.

I know that I'm going to need to regroup from my hundreds, and that's because I need to get some more tens.

I haven't got enough tens in my tens column to do six take away seven.

I'm going to cross out that four there in my hundreds for some reason.

I want to move them into my tens.

So four isn't four anymore it's three.

And I've moved one of those into my tens.

I can then go through the sums, because I've got enough in each column to do my calculations.

Five take away four is one.

16 take away seven.

Or how would you work out 16 take away seven? Well, I know that with seven, I need three more to get to 10.

And then three add six is nine.

Or, you could think, if it was 16 takeaway six, it would be 10.

So sixteen take away seven, gets me one less than 10.

I've got nine there.

And then remember that for your hundreds, it's not four anymore, because I've regrouped from it.

It's three.

So it's not four take away two, it's three take away two.

Avoiding that mistake that we saw at the beginning of the lesson.

And there you can see the exchanging and the regrouping that I've done.

So I've exchanged one of my hundreds.

I've taken it out of my hundreds column, four's changed to three, and I've moved one of those hundreds into my tens columns so that I'm able to do that sum.

When you're looking at column subtraction, it's really important to have the big number, the whole number, at the top and the part, the smaller number underneath it.

That will make sure that you're doing it in the right order and you don't make that other mistake we talked about where you're getting the part and the whole mixed up, and you're not taking the right number away.

Should we have another go? Here's another sum.

343 take away 151 and you can see I've represented everything.

Hopefully you can see again that it's in the tens column where I'm going to need to regroup, and that's because I've got a larger number in my part, five, than I've got in my whole, four.

So let's represent this again.

343.

151.

And again, like I said, we're going to exchange one of our hundreds for ten tens.

I can then go through my sum, or taking away, More would call him.

Subtraction then? one once, and then five tens, and one hundred, leaving 193.

.

Subtraction then? So I would exchange one of my hundreds for a ten.

Three has become two, and four have become 14 in my tens column.

Then, as I go through my sum, I do three take away one is two.

Fourteen take away five is nine.

And two take away one is one.

There's the regrouping and exchanging I have done.

Okay, you're going to pause the video and have a go here.

If you would like to use your online dienes, please do.

If you feel confident to just have a go straight at column subtraction, even better.

Okay, let's have a look at how you did.

You need to regroup in your hundreds to give you 15 in your tens.

You then got three takeaway two is one.

Fifteen take away eight.

How would you work out 15 take away eight? Well, I think I would split eight into five and three.

I then take away the five first to get me to ten.

I need three more gets me to seven.

Then I need remember that in my hundreds, I'm no longer doing three take away one, I'm doing two take away one.

So my answer is one.

Really well done if you saw that.

And there's the regrouping and exchanging that you've done.

Okay, let's have a look through this answer together.

Let's check it.

When you're asked to check an answer, it's a really good idea to move through it like you're answering it.

We're going to start with our ones.

question for your ones is five take away three.

Five takeaway three is two.

It looks like they've done absolutely fine in the ones column.

Let's move on to the tens column then.

Well, they should have been doing two takeaway eight, but what they decided to instead it looks like is eight take away two.

It's that mistake we talked about right in the beginning of the lesson, in the warm up.

Don't fall into this trap.

They needed to regroup in their tens and they haven't done.

Because of that, they got the answer wrong? Let's have a look at what they should have done.

They should have regrouped from their tens, changed their nine to an eight.

Move that over into their tens column to give them twelve in the tens column.

And then twelve takeaway eight would have given them two, but they should have got the right answer, 342.

Okay, time to pause the video again and have a go at your independent tasks.

Let's see how you got on.

Here are the answers to your questions.

Well done if you saw that some of them didn't need you to do column subtraction.

For example that one in the middle, 346 take away 244.

You should really have been able to do that one in your head, there was no regrouping needed.

So well done if you could see that that could be done in your head.

The last part then, there are mistakes made on these questions.

Were you able to see what they were? Well, again, for that first question there, they've just taken them away in the order that they want to.

Based on seven take away six in the ones, instead of six takeaway seven.

That's that common mistake that we talked about.

The second question, they seemed to have got a bit confused when they've exchanged one of their hundreds, they've gone from five to six.

They've actually gone up the hundred.

They should have gone down because one of those hundreds has moved in to the tens column.

Well done if you saw that that was the problem and that they made that.

And for the last question, again, they've made that mistake where they subtracted in the wrong order.

They haven't taken the part away from the whole.

They haven't done any exchanging.

Really, really well done.

If you got all of those answers and you can see those mistakes that people are making, it's clear that you really understand how to do column subtraction and how to regroup accurately to do that.

Right, it's time for your extra quiz to see how many you can get right.

Fantastic learning today.

Well done, everybody.

Enjoy the rest of your day.

Bye-bye.