Lesson video

Hello, I'm Mrs. Kaylee, and I'll be your teacher for this lesson.

So in today's lesson, we're going to use our knowledge of subtraction facts within 10 to help us to subtract within 20.

So let's have a look at today's lesson outcome.

Here's the outcome of today's lesson.

I can use knowledge of subtraction facts within 10 to subtract within 20.

Here are the key words for today's lesson.

Can you say them after me? My turn, subtraction.

Your turn.

My turn, minus.

Your turn.

My turn, difference.

Your turn.

Well done, you might have seen these words before.

Look out for them in today's lesson.

Here's the outline of today's lesson.

We're going to be using our knowledge of subtraction facts within 10 to subtract within 20.

We'll start off with some subtraction problems, and then we'll move on to using a number line.

So let's start on the learning.

Here are some children that are going to help us in today's lesson.

We've got Sam and Lucas.

Sam and Lucas have made some cakes to sell.

Can you see they've made some cakes with cherries on? First they had five cakes, then they sold two cakes.

So someone came along and bought two cakes.

How many have they got left? Can you see, first of all, they had five cakes? Sam said, "At first we had five cakes, then we sold two cakes." Can you see two cakes have been taken outta the tray? So it's five subtract two.

Two fewer than five is three cakes.

So now we've got three cakes.

Five minus two is equal to three.

"Now, we have three cakes." So there are three cakes left at the end.

Sam and Lucas have made some buns to sell.

Can you see they've made some currant buns? First they had 15 buns, then they sold two buns.

How many have they got left? Can you see we've got 15 buns on the tray? "At first we had 15 buns." "Then we sold two buns." Can you see two buns have been taken out of the tray? I wonder how many we've got left.

Can we use a subtraction fact to help us? We could use five subtract two to help us, couldn't we? That was equal to three.

"Two fewer than 15 is 13 buns." So now we've got 13 buns left.

"15 minus two is equal to 13." "Now we have 13 buns." Did you notice anything the same about the cakes and the buns? Sam and Lucas have represented their cakes and buns on 10 frames.

Can you see at the top there we've got five subtract two, that's representing the cakes.

And at the bottom, we've got 15 subtract two, that's representing the buns.

Sam said, "We had five cakes and sold two of them." Can you see I've crossed two of the cakes out? Lucas said, "We had 15 buns and sold two buns." So can you see I've crossed out two of the buns? "Five minus two is equal to three." "15 minus two is equal to 13." Can you see anything that's the same about those two? Sam said, "We could use five subtract two to help with 15 subtract two." Lucas said, "I can see an extra 10." Did you notice that we had 10 more buns than cakes? 15 subtract two is 10 more than five subtract two.

So 13 is 10 more than three.

What do you notice here? Can you see we've shown the cakes and the buns as a subtraction.

So with the cakes, we had five subtract two is equal to three.

And with the buns, we had 15 subtract two is equal to 13.

Sam said, "They look similar." Lucas said, "We are subtracting two each time." Sam said, "Five minus two is equal to three." Lucas said, "15 minus two is equal to 13." We can use five minus two to help work out 15 minus two.

Sam has noticed that 15 is 10 more than five.

We had an extra 10 buns, didn't we? Lucas has noticed that the difference is 10 more.

So first of all we had three, and 10 more than three is 13.

So we were left with 13 buns.

Sam said, "13 is 10 more than three." Lucas said, "Both differences end with three." Can you see the differences there, we've got three and 13.

They've both got a ones digit of three.

What do you notice here? We've got 15 subtract two, that's representing the buns.

"15 is 10 and five." So 15 can be partitioned into its tens and ones.

It's got one 10 and five ones.

Lucas said, "We can subtract the ones." So five subtract two.

I've put a circle round the ones Sam said, "Five minus two is equal to three." So Lucas said, "15 minus two is equal to add five minus two." So 15 minus two is the same as 10 plus three.

15 minus two is equal to 13.

Sam and Lucas are selling apples.

First they had nine apples, then they sold three apples.

How many have they got now? Sam said, "At first we had nine apples.

"Then we sold three apples." So can you see three apples have been taken out with a tray? So nine minus three.

"Three fewer than nine is six apples." So nine minus three is equal to six.

"Now we have six apples." Sam and Lucas are selling some strawberries.

They have got 19 strawberries, and they sell three strawberries.

How many have they got left? Can you see there are 19 strawberries in the tray? "At first we had 19 strawberries." "Then we sold three strawberries." Can you see three of the strawberries have been taken out of the tray? Sam said, "I know nine minus three is six." Can that help them work out this one? "So three less than 19 is 16." "19 minus three is equal to 16." "Now we have 16 strawberries." Sam and Lucas have represented their apples and strawberries on 10 frames.

Can you see the apples have been represented as green counters, and the strawberries have been represented as red counters? So we had nine apples and we subtracted three.

So I've crossed three of the apples out.

"We had nine minus three apples." And with the strawberries, "We had 19 minus three strawberries." So I've crossed three of the strawberries out.

Can you see anything that's the same with the 10 frames? Sam said, "Nine minus three is equal to six." Lucas said, "19 minus three is equal to 16." Sam said, "We can use nine minus three to help with 19 minus three." Can you see they're similar? Lucas has spotted an extra 10.

"I can see an extra 10." So there were 10 more strawberries than apples at first.

19 is 10 more than nine.

"So 19 minus three is 10 more than nine minus three." So when we look at the difference, 16 is 10 more than six.

What do you notice here? Can you see we've represented the apples and the strawberries as subtraction equations? Sam said, "They look similar." Lucas has noticed that we are subtracting three each time.

Sam said, "Nine minus three is equal to six." And Lucas said, "19 minus three is equal to 16." What's the same and what's different? Lucas said, "We can use nine minus three to help work out 19 minus three." "19 is 10 more than nine." So we had 19 more strawberries than apples at first.

"And the difference is 10 more." "16 is 10 more than six." Lucas has noticed that both differences end in six, so the ones digit is six.

What do you notice here? We've got 19 minus three.

This is representing the strawberries.

19 can be partitioned into tens and ones.

"19 is 10 and nine." That's one 10 and nine ones.

Lucas said, "We can subtract the ones." I've put a purple ring around the ones.

"19 is 10 and nine.

Nine minus three is equal to six." So when we subtract the ones, we end up with six.

"19 minus three is equal to 10 plus nine minus three." "So 19 minus three is the same as 10 plus six." "19 minus three is equal to 16." What do you notice here? Can you see we're starting with nine or 19, and we're subtracting different numbers.

What do you notice? Sam said, "They look similar." Lucas has noticed that we are subtracting one more each time.

So we had nine subtract one, nine subtract two, nine subtract three, nine subtract four, and nine subtract five.

We're going up by one each time, aren't we? And the same with 19.

19 subtract one, 19 subtract two, 19 subtract three, 19 subtract four, and 19 subtract five.

Sam said, "As we subtract one more each time," what do you notice about the answers? Lucas said, "The difference goes down by one each time." So the difference started at eight.

Nine subtract one is eight.

Then we had seven, nine subtract two is seven.

Nine subtract three is six.

Nine subtract four is five.

And nine subtract five is four.

And it was the same with the 19.

19 subtract one is 18.

19 subtract two is 17.

19 subtract three is 16.

19 subtract four is 15.

And 19 subtract five is 14.

So as we subtract more, the difference is less.

Let's check your understanding.

Which part-part-whole model can help to solve 19 minus six? So is it nine subtract five is equal to four, nine subtract six is equal to three, or nine subtract seven is equal to two? Pause the video while you think about this one.

What did you think about this one? I think the middle part-part-whole model is the most helpful.

Sam said, "Nine minus six will help to work out 19 minus six." Because 19 is 10 more than nine.

Lucas said, "Nine minus six is three, so 19 minus six is 13." Is that what you thought? Here's a task for you to have a go at.

Can you use the 10 frames to complete the subtractions? So the first one is 19 minus four, and the second one is 18 minus three.

And think about which fact can you use to help, and show it on the 10 frame.

So see if you can find a subtraction within 10 that will help you to work out the subtraction within 20.

And then look at your answers and think about what do you notice.

So you can use counters on 10 frames, or you can draw them on 10 frames to work these out.

Here's the second part of your task.

Can you use the 10 frames to solve the subtraction problems? So in part A, Sam had 18 apples and sold two apples.

How many has she got left? So you're going to work out 18 minus two.

And think about which fact could you use to help.

And then in part B, Lucas had 17 cakes and sold six cakes.

How many has he got left? So you're going to work out 17 minus six.

And think about which fact could you use to help.

So pause the video while you have a go at your tasks.

How did you get on with your tasks? Did you use the 10 frames to show the subtractions? So first of all we had 19 subtract four, that makes 15.

And the fact that you could use to help is nine minus four is equal to five.

And secondly we had 18 minus three, that's equal to 15, because eight minus three is equal to five.

Can you see we've got the counters on the 10 frames, and we've crossed out the ones that we're subtracting.

What did you notice? Sam noticed that nine minus four is equal to five, and 15 has five ones as well.

Lucas noticed that 19 minus four is equal to 18 minus three.

They both had the same answer, didn't they, 15.

How did you get on with the second part of your task, did you solve the subtraction problems? So first of all, Sam had 18 apples and sold two apples.

How many has she got left? So here on the 10 frame, I've drawn 18 counters and I've crossed out two of them.

18 subtract two is equal to 16.

Which fact did you use to help? I used eight minus two is equal to six to help me.

Can you see that 18 is 10 more than eight? So the difference, 16 is 10 more than six.

In part B, Lucas had 17 cakes and sold six cakes.

How many has he got left? So I've drawn 17 counters on the 10 frame, and I've crossed six of them out.

17 subtract six is equal to 11.

The fact that I used to help was seven minus six is equal to one.

Can you see they're similar? 17 is 10 more than seven, and 11 is 10 more than one.

Is that what you found? Let's move on to the second part of our lesson, we'll be using a number line.

Sam and Lucas have six cakes, four have cherries and the rest do not.

How many do not have cherries? Can you see here we've got the six cakes, but some of them have been covered up? Four have cherries, so how many do not have cherries? How many do you think do not have cherries, and how can you work it out? So Sam said, "There are six cakes in total." How can we work out how many do not have cherries? Sam said, "We can subtract four from six to find out how many do not have cherries." Sam and Lucas have 16 buns, four have currants and the rest do not.

How many do not have currants? Sam said, "There are 16 buns in total." Some of them have been covered up, haven't they? Sam said, "We can subtract four from 16 to find out how many do not have currants." We can use a number line to show the subtractions.

Can you see here we're showing the subtractions on two number lines? The first number line is from zero to 10, and the second number line is from 10 to 20.

So on the first number line, we're showing the cakes, six subtract four is equal to two.

On the second number line, we're showing the buns, 16 minus four is equal to 12.

What do you notice about these number lines? What's the same and what's different about the subtractions? So six minus four is equal to two, and 16 minus four is equal to 12.

Can you see that 16 is 10 more than six, so 12 is 10 more than two.

We've got a stem sentence at the bottom to help us.

Mm minus mm is equal to mm.

So mm minus mm is equal to mm.

Can you say the stem sentence with me with the numbers? So six minus four is equal to two.

So 16 minus four is equal to 12.

Is that what you thought? Sam and Lucas have six cakes, four have cherries and the rest do not.

How many do not have cherries? It was two, wasn't it? Can you see them there now? We've got six cakes and four of them have got cherries, so two have not got cherries.

Sam and Lucas have 16 buns, four have currants and the rest do not.

How many do not have currants? So 16 minus four is equal to 12.

What do you notice here? We've got two number lines showing a subtraction.

First of all, we've got nine minus three is equal to six, and then we've got 19 minus three is equal to 16.

What do you notice? What's the same and what's different about them? Sam said, "Nine minus three is equal to six." Lucas said, "19 minus three is equal to 16." Sam has noticed that they both end in six.

Nine minus three is equal to six, so 19 minus three is equal to 16.

Can you say the stem sentence with me? Nine minus three is equal to six, so 19 minus three is equal to 16.

What do you notice here? Can you see the two number lines with the subtraction on? Sam said, "10 minus two is equal to eight." Lucas said, "20 minus two is equal to 18." What's the same and what's different about them? Sam has noticed that they both end in eight.

10 minus two is equal to eight, so 20 minus two is equal to 18.

Can you say the stem sentence with me? 10 minus two is equal to eight, so 20 minus two is equal to 18.

What do you notice here? Here we've got three number lines all showing a subtraction.

What do you notice, what's the same and what's different about them? Can you see they are all subtracting two? We started with 10, 20, or 30, and we subtracted two each time, and we ended up with a number that ends in eight.

If we subtract two from a number ending in zero, will we always get a number ending in eight? Lucas thinks yes.

He said, "I think the pattern will carry on, even with larger numbers." Let's check your understanding, who is correct? Sam said, "Three minus two is equal to one, so 13 minus two is equal to 11." Lucas said, "10 minus two is equal to eight, so 20 minus two is equal to eight." Can you imagine the subtractions on the number line? Pause the video while you think about who is correct.

Who did you think was correct? It was Sam, three minus two is equal to one.

I can see it there on the number line.

So 13 minus two is equal to 11.

Lucas was not correct.

He said, "10 minus two is equal to eight." That's right.

But 20 minus two is not equal to eight, it's equal to 18.

Let's check your understanding again, who do you agree with? Sam said, "If we subtract two from a number with a ones digit of five, we will get a number with a ones digit of three." Lucas said, "If we subtract two from a number with a ones digit of five, we will get a number with a ones digit of two." Now, you could try this out on the number line.

Pause the video while you think about who you agree with.

Who did you agree with? Sam was correct.

If we subtract two from a number with a ones digit of five, we will get a number with a ones digit of three.

So Lucas was not correct, we don't get a number with a ones digit of two.

This is the fact that we can use to help us, five minus two is equal to three.

And that can be used for any subtraction where the ones are five and two.

Here's a task for you to have a go at.

Can you show these subtractions on a number line and complete the stem sentence? So the first one, we've got six minus four, and the second one, we've got 16 minus four.

So can you show those on the number lines and then complete the stem sentence? Mm minus mm is equal to mm, so mm minus mm is equal to mm.

So we're using our subtraction within 10 to help work out the subtraction within 20.

Here's the second part of your task.

Can you show these subtractions on a number line and complete the stem sentence? So here we've got 10 minus five and 20 minus five.

Can you show those on the number line and complete the stem sentence? Here's the third part of your task.

Can you solve these subtraction problems using a number line? So Sam said, "I had seven apples and I ate three apples, how many have I got left?" Lucas said, "I had 17 strawberries and I ate three of them, how many have I got left?" So can you solve Sam's problem and show it on the top number line, and then solve Lucas's problem and solve it on the bottom number line.

And see if you can complete the stem sentence.

Pause the video while you have a go at your tasks.

How did you get on with your tasks? Did you show the subtractions on a number line? So first of all we had six minus four, that's equal to two, and I've shown it on the number line.

Then we had 16 minus four, that's equal to 12, and I've shown that on the second number line.

Here's our stem sentence.

Six minus four is equal to two, so 16 minus four is equal to 12.

Here's the second part of the task.

Did you show these subtractions on a number line? So first of all, we had 10 minus five is equal to five, and then we had 20 minus five is equal to 15.

Did you show those on the number line? Let's say the stem sentence together, 10 minus five is equal to five, so 20 minus five is equal to 15.

Here's the third part of the task.

Did you solve these subtraction problems using a number line? So Sam said, "I had seven apples and I ate three apples, how many have I got left?" Can you see that on the top number line, seven minus three is equal to four.

Then Lucas said, "I had 17 strawberries and I ate three of them, how many have I got left?" And I've shown that on the second number line, 17 minus three is equal to 14.

So she had four apples left, because seven minus three is equal to four.

Then Lucas said, "I had 17 strawberries and I ate three of them, how many have I got left?" Can you see that on the second number line? So 17 minus three is equal to 14.

Let's say the stem sentence together, seven minus three is equal to four, so 17 minus three is equal to 14.

We've come to the end of our lesson.

Today we were using our knowledge of subtraction facts within 10 to subtract within 20, and this is what we found out.

Subtraction facts within 10 can be applied to subtraction facts within 20.

For example, if five minus two is equal to three, then 15 minus two is equal to 13.

A number line can be used to show the link between subtraction of one digit numbers and teen numbers.

So that's when we are looking at our subtraction facts within 10 and using them to help us with our subtraction facts within 20.

And we can use this stem sentence to help us, if we subtract mm from a number with a ones digit of mm, we will get a number with a ones digit of mm.

Well done, everyone, see you soon.