Lesson video

Hello, everyone.

I'm Miss Brinkworth.

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

Let's have a look at that learning objective again.

We're going to be subtracting multiples of 10 from three-digit numbers.

We're going to be looking at the mental strategies we can use to do that really quickly, but accurately in our heads.

Our lesson agenda is that we're going to look at how a part/whole model can make it really clear what the question's all about.

We're going to practise using our known facts.

We know lots about adding and subtracting already so we can use those to help us in today's lesson.

We're going to look at regrouping and how that helps us when we have to subtract larger numbers.

And then you'll have your independent work which will give you a chance to practise these new skills.

And that exit quiz will allow you to see how well today learning's gone in.

Great, hopefully you've got what you need.

Let's get started.

So here's a little warm up.

Some of these are true and some of them are false.

So the number that you're focusing on is 421.

And you just need to look at these statements and decide whether they are the same as 421 or not.

What do you think? Pause the video and take as long as you need.

Well, how did you get on? In fact, it's just one of these which is false.

All of the others are true.

So well done if you could see that that was the only false one.

And that's because there aren't 3 tens in 421, there are 2 tens.

Well done if you could see that.

The others all have a big tick.

Okay, so we talked about how, when we look at subtraction, a part/whole model can be really useful.

This is what a part/whole model looks like when we are subtracting.

So we've got the whole, which is there, 362 and we've got one of the parts.

We need to find the other parts.

So in this part/whole model, we can see what we already know and we can see what's missing.

That's where our answer is going to go when we've solved the calculation.

So this calculation here looks like this.

Now remember with subtraction, it does matter which order you do it in.

It's got to be the bigger number first.

So try and remember that as we go through today's learning.

Okay, so how will we answer 362 take away 20? Well, what we're going to be looking at is that tens column.

And that's because today's learning objective is about subtracting multiples of 10.

So for this first set of questions we're just looking at amending the tens column.

So if you see, what we're looking at is 60 subtract 20.

60 subtract 20.

Well, the fact that we can use to help us there is 6 take away 2 is 4, so 60 take away 20 is 40.

So that tens column changes from being six to being four.

And well done if you could see that.

Let's just go back for a moment.

Here we are again with our part/whole model with a new question, 284 take away 40.

Now we can use our dienes to help us with this question.

So if you've got some online dienes, have a go at guessing them to make this question.

Here they are, there's my 284 take away 40.

Have a go at getting your dienes to make this question.

Okay.

And then when we using our dienes it's really clear to se that what we need to do is take those four tenths away from that answer.

Instead of 284, eight take away four is four, so we'll have 244 and that finishes that part/whole model for us.

We've got two parts now, 244 and 40 is 284.

Here's a question that's going to be slightly different.

We've got a part and a whole.

It doesn't matter which one of those boxes we put our part in.

We've got 436 and we're taking away 30.

What we're looking at, really, is three take away three.

And remember that if I've got three and I give away three, I've got zero left.

So 30 takes away 30 also gives us zero in the tens column.

So that means my missing number is 406.

And when we look to in a part/whole model that becomes really clear.

Of course, 406 add 30, it's 436.

So when I add my parts together, I've got my whole.

Okay.

Your turn then.

Pause the video here and have a go at using that strategy to answer this question.

How did you get them? Let's have a look.

If you were stuck, this is the sum that's there to help you.

Three take away two, so 30 takeaway 20.

Well, hopefully we're really confident that three takeaway two is one.

So 30 take away 20 is 10.

And so we're left with 715 in our tens column.

It's gone from three to one, 715.

Now this question is going to be slightly different.

Have a look and think about what's different about this question.

Well, hopefully you can see what's different is that the number we're taking away, 50, has got a bigger number in the tens column than the number we're taking away from.

Now, what sometimes happens on questions like this is people decide to do, they just take the smaller number away.

So the wrong answer that you might see quite frequently for this question is 210.

And what people have done is they've taken 40 away from 50.

And I completely understand why they do that, but that's not the question.

The question is 240 takeaway 50.

It's got to be done in that order and you can't change it just 'cause it will be easier to take 40 away from 50.

I completely understand.

But there's a really simple way of doing this.

So, what we'll do is we'll split 50 up into 40 and 10.

Why do you think I picked 40 and 10 split my 50 up into? Well that's because I need to take away the whole 50, but if I take away the 40 to begin with, that's going to be nice and simple 'cause that gets me down to 200.

I then just need to take away 10 more to get me to 190.

So that's my answer for that question.

Have a go at this one.

And if you're stuck, think about this, 40 or four can be split down into three and one.

So 40 can be split down into 30 and 10.

But this question, I think it'll work really well if you subtract 30 first and then subtract 10.

Pause the video and have a go.

How did you get on? Maybe some of you didn't need that help but if you take, sorry, if you take the 30 away first, it gets you down to 505.

And then if you take the 10 away from that, you get 495.

Well done if you've got that right answer.

Here's some questions.

Pause the video and have a go at using those strategies we've talked about to answer these questions and mentally.

How did you get to one, everyone? I'm going to put the answers up.

I'm not going to go through them in too much detail.

Well done if you got to question four and you've got that question right.

Or even if he gave it a go, 'cause that's a tricky one which bridges into the hundreds that we've been practising.

But you have really challenged yourself to answer that question so well done if you had a go at question four.

And question five, sorry, in particular.

Well done.

Okay.

It's that time for some independent work now.

So pause the video here.

Take as long as you need.

And we'll come back together for the answers.

Let's see how you did.

So for this question, I've asked you to take a three-digit number and a multiple of 10 and subtract one from the other.

I can't give you all the answers 'cause I don't know which ones you picked but you can have a go at checking the answers yourself.

What you could do is use the inverse.

So you could take your answer and your multiple of 10, add them together and you should end up with that original three-digit number.

That's a really good way of checking your answers.

That's how adding and subtracting are really useful to know that they're the inverse because we can use them to check our answers.

But if you had a go to the challenge and you had a go at matching those two numbers together to get these three numbers at the bottom here, well done.

I can give you some answers to these.

I don't know that needs to be only correct answers but these are the ones I found.

So if you were looking for 380, you could do 400 take away 20.

If you were looking for 232, you can do 312 take away 80.

And if you were looking for 488, well then if you saw you could do 578 take away 90.

If you challenge yourself and got some of those answers right, really well done, 'cause that's quite a challenge.

Finally, let's have a look at these statements here.

Is it always, sometimes or never true, that when you subtract a multiple of 10 from a three-digit number, only the tens digit changes.

Well, hopefully you could see from the learning that we've gone through in this lesson, that is sometimes true.

On a question like this, it's only the tens column that changes because the number that we're taking away, three, is bigger than the number we started with, four.

And so it's just a tens column that changes.

But that's not always true because if we have a smaller value in our tens columns than the amount we're taking away, a hundreds column will also change as we are regrouping.

What about this then? Is this correct? You don't need to answer it, but I'll be really please If you could just see by looking at it that actually it's not correct.

I know straight away that it's not correct, my bet you did as well because the numbers got bigger.

Two, two, seven, 227 has somehow turned into 287.

The number shouldn't have got bigger.

It should have got smaller because we're subtracting.

So well done if you could see that answer was not correct.

And that's the right answer.

If you got that, you're really pushing yourself.

Well done everybody.

Okay.

It's time for the quiz to see how well today's learnings going in.

Really well done everybody, on some fantastic learning in today's lesson.

Bye bye.