Lesson video

Hello.

My name is Mrs. Behan.

And for this lesson, I will be your teacher.

In this lesson we are going to use everything we know about subtraction to do some mental calculations.

We use subtraction all the time in our daily lives.

We might try and work out how many points ahead of our friends we are in a game.

Or we might workout how many sweets we have left when we've given some out.

We might even try and compare temperatures from one day to the next and finding the difference to help us.

So let's begin looking at subtraction together.

Let's begin by having a look at the lesson agenda.

Throughout the lesson, we will use a range of subtraction strategies.

We will decide on the most efficient strategy.

Then we will practise subtraction strategies ourselves.

And at the end of the lesson, there will be an independent task for you to have a go at.

I know you became to find out how you got on, so I will make sure I go through the answers with you.

There are a few things that might help you in this lesson.

You'll need something to write with.

So a pencil or a pen, something to write on and to help you with some calculations, if you need them, something that will represent dienes.

Here you can see I have 10 spaghetti sticks to represent 100, one spaghetti stick to represent tens.

And one piece of twisted pasta to represent ones.

If you don't have those things to hand, just pause the video here whilst you go and get them.

Remember to try and work somewhere quiet, where you won't be disturbed for the lesson.

Let's get stuck into looking at subtraction strategies.

So the first thing I'm going to do is just to remind you of some strategies that you will have probably come across in the past.

Partitioning and subtracting.

So this is way we partition numbers and take something away.

Using known facts.

So I might use three subtract two is equal to one.

To work out the 30 subtract 20 is equal to 10.

Subtracting from the whole and partitioning one part.

We will come to that later on and counting on to find the difference.

I always say, when you look at two numbers, can you work out what the gap is between those numbers? That's called finding the difference.

So difference means the gap between the numbers.

So I would like you to look at the calculation on screen, and you're going to use your pen and your paper just to do some working out, to see if you can calculate, 90 subtract 56.

Pause the video here whilst you have a go.

Remember there's no one set strategy.

Okay then let's have a look through.

So like I said, there's not one set strategy that you should use.

However, there are strategies that are more efficient than others and to be efficient, that means to do something well, go quickest way, and the best way, rather than the long way round.

So as I go through these now, I'm going to look at some different strategies.

And what I would like you to do is to think about how efficient they are.

Everyone watching this video now might have come up with a different way.

And that is absolutely fine, but we're just going to focus on the strategies coming up.

and remember your job is to think about how efficient the strategies were.

So this is Anna and Anna would like to use the subtracting from the whole, partitioning one part strategy.

So let's see how she gets on with this.

She first says, I partitioned 56 into tens and ones.

I subtracted the tens.

I subtracted the ones.

So you can see she's done some different steps here.

So on the screen to the left, you can see the calculations that Anna did.

56 is equal to 50 plus six.

And it's important that she partitioned those numbers correctly.

Then she was able to subtract 50 from 90, which left her a difference of 40.

And then she subtracted six from 40, which left her with 36.

So read these calculations on the screen with me, 56 is equal to 50 plus six.

90 subtract 50 is equal to 40 40 subtract six is equal to 34.

So Anna knows that 90 subtract 56 is equal to 34.

What did you think about her strategy? Is that something that you would have done? So this is Junaid And Junaid is going to count on to find the difference.

The difference means the gap between the numbers.

We need to remember that.

Junaid says, I know that the whole is 90 and one part is 56.

So he's thinking about the relationship between the wholes and the parts here, which is really, really good.

To find the difference I need to add on four to get to 60 and then add 30 to get to 90.

The part is 34.

So Junaid is not actually starting with 90.

He's going to start with 56, but let's see how he represents his calculations.

So Junaid has created a number line and it's marked on 60.

It also put 56 here at the beginning because that's the part that he knows the value of one part is equal to 56.

So now he needs to try and find the gap between those two numbers.

60 is a bit of a milestone.

It's a nice multiple of 10 that he will use to help him work out the difference.

So Junaid starts by looking at 56.

He adds on four to make this lovely multiple of 10.

So he gets to 60.

Now he has to work out the difference between 60 and 90, which calculates to be 30.

Now Junaid needs to put those two numbers back together.

Those two values four and 30, he calculates 30 plus four is equal to 34.

What did you think about Junaid's strategy? Do you think that was easier than Anna's? So whose strategy was most efficient? Let's remind ourselves of the strategies.

Anna was subtracting from the whole and she partitioned one part.

She partitioned 56 into 50 and six.

And Junaid counted on to find the difference.

Which strategy do you think was the most efficient? Well, it's important to know that when numbers are closer together, it is easier to count on to find the difference.

So in this instance, I think Junaid's strategy was perhaps easier.

Pause the video here whilst you have a go calculating, 39 subtract 21.

Remember there's no one set strategy, but there are more efficient strategies than others.

Okay.

So you've had a go.

Which strategy did you use? Did you use one that we'd already looked at? or perhaps something else? Let's have a look to see what Zara used.

Zara is partitioning and subtracting.

She said, I can subtract the tens, then subtract the ones.

So Zara has noticed, first of all, that there won't need to be any regrouping or exchanging here.

So Zara's method is, to look at the tens first.

She's got her whole here of 39, 30 subtract 20 is equal to 10.

So she's taken 30 put them in this group.

She's been left with one 10 here.

Then Zara has subtracted the one.

Nine subtract one is equal to eight.

So she had nine here, but has removed one and put it over here.

That makes the 21.

So whatever she has left is the size of the other part.

So Zara now has 18 left.

39 subtract 21 is equal to 18.

What did you think of Zara's strategy? Was that efficient? Did she need to use the dienes as well to prove what she had calculated? Let's have a look at Anna's suggestion.

Anna is partitioning and using known facts.

This'll be an interesting one.

She said, I know that three subtract two is equal to one.

So 30 subtract 20 is equal to 10, nine subtract one is equal to eight.

So 39 subtract 21 is equal to 18.

So here are her calculations.

So she's used three subtracts two is equal to one.

Again she's noticed that we won't have to do any regrouping or exchanging because nine has a greater value.

Nine ones has a greater value than one one.

So there's no exchange or regrouping needed.

So she can use this method.

She then goes on, nine subtract one equal to eight and puts them back together.

She recombines those parts, 10 plus eight is equal to 18.

What did you think of Anna's method? We've got one last method.

So this is Junaid and he's sticking with his counting on to find the different strategy.

He says, I know that the whole is 39 and one part is 21.

I need to find the difference to sorry, To find the difference, I need to add on 10 to get to 31 and then add eight to get to 39.

The part is 18.

So let's have a look at his number line.

So is marked on 39.

He's got 21 on there as well He's using 31 as a marker because he knows 21 plus 31 if he counts on is ten difference between 21 and 31 is 10.

What is the difference between 31 and 39? That's right.

It's eight.

Puts those parts together and he knows that the difference between 39 and 21 is 18, which of our friends strategies, do you think was the most efficient? Anna who used the partitioning and known facts strategy? Junaid who used the counting on to find the different strategy? and Zara who partitioned and subtracted? Out of the three for the calculation that we were given, I personally thought Junaid strategy was very good.

I thought it was easier to count on to find the difference for this calculation.

Here is a reminder of just some of the strategies that we can use when working with subtraction.

Partitioning and subtracting, using known facts, partitioning from the whole, subtracting from the whole, partitioning one part, counting on to find the difference.

I'd like you now to pause the video here and have a go at calculating 80 subtract 48, think carefully about the sort of strategy that you use.

Try to calculate 80 subtract 48 in different ways, using different strategies.

When you ready come comeback and I'll show you my ideas.

I tried out four different strategies and here they are.

The first one I tried was counting on to find the difference.

So the calculation was 80 subtract 48.

So I put on a number line 80 and the smaller, the number, which was 48.

I needed to find that gap.

I added two to get me to 50, a nice multiple of 10, and then to get from 50 to 80, I needed to add 30.

I add these two parts together to work out that the difference between 80 and 48 is 32.

I had to go the partitioning and subtracting strategy.

So you can see here that I have partitioned 48 into 40 and eight.

80 takeaway 40 is 40.

40 subtract eight is equal to 32.

So 80 subtract 48 is equal to 32.

So I took the whole, subtracted the tens, then subtracted the ones.

I had to go using known facts.

I know that eight subtract four is equal to four.

So 80 to subtract 40 is equal to 40.

I know I had eight left to take.

So I did 40 subtract eight, which equals 32.

So 80 subtract 40 is equal to 32.

My last strategy was rounding and adjusting.

So I didn't round of 80 because that's a lovely multiple of 10 already, but I rounded 48 up to 50.

So then I calculated 80 subtract 50, which equals 30.

Now I subtracted more than I actually needed to.

I subtracted two to many.

So I had to add those two back up.

So 30 plus two is equal to 32.

So that was my rounding and adjusting strategy.

Which strategy did you use? Was it the most efficient one? Which of my strategies do you think was the most efficient? I perhaps would have used counting on to find the difference strategy, who was the only one that I was going to use.

I do prefer that strategy, but it does depend on the numbers that you have got.

Now it's time to have a look at the independent task using the digits two, five, seven, and eight, how many possible answers can you find? Which strategy will be most efficient to solve the calculations you create? You might take seven and eight, for example, and you might create 78.

You might then subtract 25 because you built 25 using your two and five.

So your calculation is 78 subtract 25 and you find the difference and write it here.

Here is a space for you to use when writing down your subtraction calculations.

Pause the video here to complete your task.

When you're ready, come back to me and we will look at the answers together.

Okay then.

So I'm sure you'll have used so many different subtraction strategies to find out the answers or the solutions to this problem.

I'm going to show you two calculations at a time.

You might want to have a look on your sheet that you've written down and see if you can find these calculations.

82 subtract 75 is equal to seven.

I filled in zero there because I had a blank space.

So I put zero in there to show I have zero tens.

58 subtract 27 is equal to 31, 85 subtract 72 is equal to 13, 87 subtract 52 is equal to 35, 72 subtract 58 is equal to 14 75 subtract 28 is equal to 47, 82 subtract 57 is equal to 25, 78 subtract 25 is equal to 53 78 subtract 52 is equal to 26 85 subtract 27 is equal to 58 57 subtract 28 equals 29, and 87 Subtract 25 is equal to 62.

If you'd like to, please ask your parents or carer to share your work on Instagram, Facebook, or Twitter tagging @OakNational, @LauraBehan21 and #LearnwithOak.

I hope now that whenever you're doing a subtraction calculation, you will think about which is the most efficient strategy to use.

See you again soon.

Don't forget to take the quiz.

Bye bye.