Lesson video

- Hello everybody, and welcome to another math session.

My name is Miss Hughes, and today we're going to be looking at subtracting two 2-digit numbers.

So let's get going.

This is our lesson agenda for today then, guys.

We are going to start off by subtracting two 2-digit numbers, then you are going to have a Talk Task.

Next, we're going to think about finding new number facts that are going to help us subtract two 2-digit numbers.

And finally, it's your main task and of course your quiz at the very end.

For today's lesson, you are going to need a pencil and rubber, and some paper.

So pause the video now to go and get these things if you have not got them with you already.

Fantastic.

Let's move on.

To warm our brains up at the beginning of this lesson, we're going to start with solving these equations by using our known facts to help us.

So you have a few 2-digit numbers that you are going to add together, and before you do that, I want you to think about the known facts within 10, that you have that can help you out the tens and then the ones so that you can do this mentally.

Pause the video now to have a go at this task.

Okay, let's go through those answers then, guys.

So let's start with looking at the tens and the known facts that we could use to help us add the tens together.

26 add 32, I know that we've got 2 tens and 3 tens.

So the number fact 2 + 3 = 5 is going to help me to add my tens together.

3 tens add 6 tens is 9 tens.

3 + 6 = 9 is going to help me with this one.

Add 5 tens here and 4 tens here.

So 5 + 4 = 9 is going to help me with that one.

And 4 tens here add 2 tens here.

4 + 2 = 6 is going to help me with this answer here.

So let's have a look at the answers we should have got.

58, 99, 96, and 68.

There were also some known number facts that would have been useful for adding your ones together.

So 6 + 2, we know is equal to 8.

So that was going to help us add our ones.

2 + 7 is 9, that was going to help us add our ones here.

We know that 5 + 1 = 6.

And we know that 4 + 4 = 8.

So those number of facts were going to help us add our ones and these number facts help us to add our tens.

Well done if you've got those and the correct answers.

Let's move on.

We're going to think now about some of the known facts that we can use to help us solve subtraction problems like this one.

So let's have a read of this problem that we've got.

On Monday, 75 people were in town.

24 people left for work.

How many people were still in town? Let's use a part-whole model to help us work this out.

Remember when we're doing a word problem like this, we need to think about what do we know, what facts do we know, and what values do we know of our part-whole model, and what values are we trying to work out.

Hmm.

Well, I can see that my whole is 75, because we start with 75 people.

So that's my whole.

And I can also see that one of my parts is worth 24.

So the value of one part is 24.

So let's see what that looks like.

My whole is 75 because 75 people are in town.

Then 24 people leave for work.

So I'm going to put 24 people in here.

To work out this missing value here, I'm going to need to take my whole 75 and take away 24.

So the equation for that would be 75 - 24 and that will give us our missing part.

We can use our known number facts to help us work out this problem: 75 - 24.

So I'm going to partition my whole, 75, into 7 tens, which is 70, and 5 ones, which is 5.

And I can partition my first part, 24, into 2 tens, which is 20 and 4 ones, which is 4.

Now that I've partitioned both our parts, of our whole and our parts, I can think about working with our tens and then our ones.

Let's look at our tens first.

In my whole, 75, I've got 7 tens and in my part, my first part, 24, I've got 2 tens.

So if I know that 7 - 2 = 5, then I also know that 70 - 20 = 50.

So I know the value of my tens is going to be worth 50.

Now we can look at our ones.

If I know that 5 - 4 = 1, then I know that my value of my ones is going to be 1.

Because I've got 5 ones, I'm taking away 4 ones, and I've got 1 left over.

So now I know that my last part is going to have 5 tens, which is 50, and 1 one, which is 1.

50 add 1 is 51.

So my answer is going to be 51.

Let's have a look at another one now.

On Tuesday, 85 people were in town and 24 people left for work.

How many people were still in town? So let's use a part-whole model to help us.

We need to figure out what parts of our part-whole model that we have and what is missing.

I can see from my question that I'm starting with 85 people in total.

So I know that 85 is my whole.

So let's put 85 in the whole.

And I know that one of my parts is 24.

So one of my parts is being taken away from my whole is 24.

So let's put 24 in there.

So I'm going to do the equation, 85 - 24, and that is going to help get us our missing parts here that we don't know.

In other words, we're doing 85 - 24, which will give us our missing part.

Remember, we can use our known number of facts to help us work out this equation mentally.

So our whole is 85 and one of our parts is 24.

So I've partitioned them into tens and ones.

85 has 8 tens, which is 80, and 5 ones, which is 5.

24 has 2 tens, which is 20, and 4 ones, which is 4.

Let's look at our tens first.

If I know that 8 - 2 = 6, then I know that 80 - 20 = 60.

So I know that in my missing parts, I'm going to have the value of 60 in there because I've got 6 tens leftover.

Now I can look at our ones.

I've got 5 ones here in my whole and I'm taking away 4 ones.

And I know that 5 - 4 = 1.

So I'm going to have the value of one also in my missing part.

So now that we know what the value of our tens is going to be in our missing part and the value of our ones is going to be in our missing part, we can add them both together to find out what are missing part is worth.

So, 60 + 1 = 61.

So my answer to the whole equation is going to be 61.

85 take away 24 is equal to 61.

Okay guys, it's now your time to have a go at this in a Talk Task.

So you've been given six different equations where you are subtracting a two-digit number from a two-digit number.

And I want you to use your number bonds within 10 to help figure out the answer and to do this mentally.

So you need to think about the number of bond that is going to help you subtract the ones and the number bonds that are going to help you subtract the tens and find the new fact that's going to help you with.

Let us go through an example together, and then I'm going to let you get on with the rest yourself.

We're going to look at this first one: 64 - 52.

So I'm going to start by writing out my equation.

64 take away 52, and I don't know yet what my missing part is so I'm going to put a question there.

But I know my whole I'm taking away one part.

Okay, let's have a think about our number bond for the ones then.

I can see that I've got 4 ones in the number 64 and 2 ones in the number 52.

So the number bond for the ones is 4 take away 2 is equal to 2.

So I know that in my missing parts I'm going to have 2 ones.

Now we can look at our tens.

Let me put squiggly line onto those ones.

In 64, I've got 6 tens and in 52, I've got 5 tens.

So my number bond for the tens that is going to help us is 6 tens take away 5 tens is equal to 1 ten.

So I know that I've got 1 lot of 10 and 2 ones in my missing part.

So now that we have a number bond for the tens, we know that 60, because that's 6 tens, take away 50, because 5 tens is worth 50, is going to give us 10, because 1 ten is worth 10.

Now, all I need to do is add together my tens and my ones.

So I've got 10 + 2 which is 12.

So the answer to my equation is 12.

Okay.

Pause the video here now to have a go at your Talk Task.

Remember to structure your sentences in this way.

We do number bonds to ones and your number bonds to tens so you know what number facts you are deriving and how they fit together.

Play the video when you're ready to continue.

All right guys, it is time for your independent task now where you are going to be subtracting two-digit numbers.

So I would like you to complete the equations that are on your sheet by using known facts to derived the new facts that you're going to be adding together.

So.

When you are doing that, I would like you to complete part-whole models pictorially.

So I want to see you drawing your tens and your ones really clearly.

As you're going through your worksheet, I also want you to see if you can spot any patterns in the numbers.

Let's have a look at an example first, and then you can get on with your task.

So my example here is 79 - 25.

So I'm taking away 25 from my whole 79.

Because I know that my whole is 79, I've drawn that out using tens and ones in my whole.

Okay.

Now I want to think about my number bonds that are going to help me to work this out.

So starting with my ones, I've got 9 ones in 79, or 9 ones in my whole, and 5 ones in 25.

I know that 9 take away 5 is 4.

So 9 ones take away 5 ones is going to leave me with 4 ones in my missing part.

But in my part-whole model, I can represent that I've taken away 5 from 9 and I put it in my part and I'm left with 4 ones here.

Now, moving on to the tens, I start off with 7 tens, which I've drawn here, and I'm taking away 2 tens.

I know that 7 - 2 = 5.

So let's see what that looks like.

There we go.

And the two belongs in this part because remember one of my parts is 25.

So now I'm left with 5 tens.

Remember, if 7 take away 2 is 5, or if I know that 7 take away 2 is 5, then I know that 70 - 20 = 50.

So now we know that our missing part is worth 50 and 4 ones.

50 and 4 ones is 54.

So let's move them there and our ones in there.

So my missing part and my answer to this equation is therefore 54.

Pause the video now to complete your task and resume the video once you are ready to continue.

Okay, let's look through those answers.

So 56 take away 25 is 31.

56 take away 35 is 21.

56 take away 45 is 11.

56 take away 55 is 1.

56 take away 15 is 41.

66 take away 15 is 51.

76 take away 15 is 61.

And 86 take away 15 is 71.

Remember we asked you to spot any number patterns as you were going through this worksheet.

You might have noticed that my tens in this part increases as my tens increase in this part of my equation, my answer decreases because I'm taking away more.

Let's have a look at the other answers.

We have 54, 53, 52, 51, 54, 55, 56, and 57.

The pattern isn't so much in the tens now, but it's in the ones.

So you might have noticed that in this set of equations, my number of ones in my whole decreases.

Therefore, the number of ones in my answer is also decreasing.

Let's have a look at these ones.

Here, the numbers in my part, or the ones that are in my part, or the part that I have is decreasing.

This means that because the number that I'm subtracting is getting less and less or smaller and smaller, my answer is increasing.

It's getting larger.

So those are the patterns.

Well done if you spotted them, and well done if you've got answers correct.

Team, you did such an excellent job with your learning today.

I was really impressed with how you were able to subtract two 2-digit numbers and the mental strategies that you used for that.

Hopefully, see you on another session soon.

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

All there's left for you to do now, team, is complete your quiz.

So when the video is ended, don't forget to do that to see how much you have remembered from today's lesson.

Good luck!.