# Lesson video

In progress...

Hello mathematicians, Miss Chalrlton here and Hedwig back for some more exciting maths.

Let's see what we're going to be doing today.

So we're going to subtract a one digit number from a teen's number, using a known fact.

A teen's number like 13, 14, 15, 16.

So the numbers are getting bigger now.

We're going to learn how to use number bonds when subtracting teen numbers.

And then identify and solve subtraction equations using those number bonds.

So using what you already know to be able to solve some trickier questions.

Then you'll do your independent task and the end of lesson quiz, You're going to need a pencil and some paper.

♪ Hands up star words ♪ ♪ Number bond, ♪ ♪ Known fact ♪ ♪ Subject ♪ I wonder how many times they'll come up in today's lesson.

Look at all these presents and those elves.

Now, do you think these elves a sorting those presence out properly, or do you think they're being a bit silly and playing with them? They look a little bit mischievous to me.

What do you think? We're going to use this picture today to create some equations to help us solve them.

So the first problem that we've got is that these elves were naughty.

They wrapped 18 presents.

Then, then one sneaky elf broke into the store cupboard and stole five presents.

Isn't that naughty.

We shouldn't steal presents.

So we had 18 presents all wrapped up and ready to go.

And then the naughty elf stole five.

How many presents do they have left now? Well, if some of the presents were taken, that means that the numbers are getting smaller.

So the operation that we need to do is subtraction.

Could everyone show me, subtract.

You do a little one or this one.

Subtract or take away.

A little trick to remember is if you see the words, how many do they have left now, if you see that word, left it often means that you need to do a subtraction equation.

Let's have a look how we could do this.

So we could do it like this with our 18 presents and we cross off the five presents that were stolen And we could use a number line to help us.

But, the problem is if we we do it like that, because we're doing so much counting, it means that we, we are much more likely to make some mistakes.

So what we're going to do today is learn a trick, a strategy of how we can subtract nice and quickly, and speedily without the chances of making mistakes.

Are you ready to try this new strategy? So what we do is we create an 18 presents.

Can you see, I've grouped them into 10.

I've made a group of 10 and then I have eight one's.

18 presents is one, 10 and eight ones.

18.

Then I need to take away five.

But that's a bit tricky to do that.

18 subtract five.

We're getting into teen numbers there.

But instead of counting each individual one, I could use the knowledge that I already have in my brain to help me solve that.

Can you see that We've got our keywords there or our star words, known fact, subtract.

I can use my known facts to help me subtract like this.

I can see the 18 has got eight ones and I'm taking away five ones.

Now that's tricky, but I know that eight subtract five is equal to three.

Let's have a look at that with cubes.

Eight subtract five is equal to three.

So 18 subtract five is equal to 13.

Let's look more closely.

In the first representation with the cubes.

I can see I've got eight ones.

I've got eight yellow cubes one, two, three, four, five, six, seven, eight, eight yellow cubes.

And I'm taking away five one, two, three, four, five.

In the second representation.

I've got 18 cubes.

so I've got a group of 10 and then eight ones.

The only difference between these equations is that 18 has got a 10 in it.

It's got one 10.

But it's got the same number of ones.

So I use what I know.

I know eight subtract five.

And I use that to help me solve tricky equations, like 18 subtract five.

Let's try again, with another example.

This time it's chocolates.

we've gone from present to chocolate.

we are lucky today.

We need to think which bonds or which known facts can we use to help us solve this equation.

So we've got 18 pieces of chocolate there, I can see a group of 10, 11, 12, 13, 14, 15, 16, 17,18 18 pieces of chocolate.

Now how many pieces of chocolate have been eaten? Who do you think crept along to eat those? Do you think it was the elves again? Or do you think maybe it was sneaky Hedwig? Do you think she sneaked along? I'm not sure because I don't think Elves like chocolate.

Maybe it was those elves.

So we've got 18 pieces of chocolate and somebody has eaten four of them.

They've taken four away and gobbled them all up.

So we need to think about what known fats we can use to help us solve that equation.

Because 18 subtract four is tricky.

But what we could do is first of all, find out group of ten.

As a group of ten and our ones presents.

Those are 18 altogether.

Then four were eaten.

So cross off the four.

So the new, the known fact that I can use to help me is eight subtract four.

Eight take away four, which means that I complete the whole equation of 18 subtract four.

If I know eight subtract four is equal to four.

Then, I know that a 18 subtract four is equal to 14.

It's the same number of ones, just a different number of tens.

Should we try again? See if you can have a go with this, but don't worry I'll help you.

So now we've got some eggs.

And this time we've smashed them.

The first thing I need to do is do what? Find a group of ten.

Well then, that's group of 10 eggs.

And then we'll put the rest of them underneath.

So I've got a group of 10 and then one, two, three, four, five, six, seven, eight.

So I've got 18 eggs again.

Then how many eggs were smashed? One, two, three, four, five.

Five eggs have been smashed.

So 18 takeaway five is tricky, but what do we do? We use what we know.

We're using known fact.

We know, as we've just done before.

Eight subtract five is equal to three, which means that, 18 subtract five is equal to 13.

Let's try saying that together.

If I know eight subtract five as equal to three, then I know a team subtract five is equal to 13.

Really, really great mathematical languages there everybody.

Shall we have a celebration.

Maybe we should have a hulk clap today because we're doing so well.

You're ready for the incredible hulk.

Hulk.

Really well done.

What I want you to do is have a look at the equations and choose which number bond, which known fact will help you solve the trickier equation.

So your first teen equation is 18, subtract seven.

But in that blue box, there's an equation, there is a known fact that will help you solve that.

See if you can match up the ones that will help you, and then you can solve the equations.

Pause the video now and then come back when you're finished and work through the answers together.

Right? Let's see how we got on, shall we? So the first equation was 18, subtract seven.

That's tricky.

But I do know that if I do eight subtract seven, Look.

Eight subtract seven is there to help me.

Eight takeaway seven is equal to one.

So, 18 takeaway seven is equal to 11.

Because 18 has a 10 in it and eight doesn't.

Now let's try 18 takeaway six That's tricky, but I know eight takeaway six is equal to two.

So 18 take away six is equal to 12.

And the last one, 18 takeaway five, that's tricky.

But eight subtract five is equal to three.

So, 18 subtract five is equal to 13.

Can you see how we use all of our known facts to help us solve more difficult equations.

Are you ready to explain to Hedwig with what we did? Wakey wakey, Hedwig.

Say wakey, wakey.

Wakey, wakey.

Right Hedwig.

We had a really challenging lesson today with some subtraction.

What did we do first? Well, we needed to solve some tricky equations using teens numbers.

And we found that was quite hard.

What I found it quite tricky anyway.

So what I did was that I used what I already knew to help me solve the equation.

For example, if eight subtract, if eight subtract five is equal to three.

Then 18 subtract five is equal to 13.

Do you understand? She's not sure.

Let's have another little think.

Let me think.

We just had to look at how many tens there were and how many ones there were.

If we notice that the ones are the same, that can help us do the equation.

So if I know that 18 subtract three, if 18 subtract three is a bit tricky.

I could do eight subtract three, which is equal to eight, seven, six, five.

which means that 18 subtract three is equal to 15.

Is that a little bit clearer now? She wasn't sure today because it was such tricky learning, but I think she got there in the end.

And hopefully you did as well.

Off we go now.

Do your quiz and see you again very soon.

Bye, bye.