Lesson video

Hello, my name's Mrs. Hopper and I'm really looking forward to working with you in today's maths lesson.

It's from our unit adding and subtracting ones and tens to and from two-digit numbers.

So, we're going to be looking at two-digit numbers and we're going to be thinking about adding and subtracting and hopefully using our known facts to help us.

So, if you're ready, let's make a start.

So, in this lesson we're going to be using our number facts to subtract multiples of 10 from a two-digit number.

So, have you got your number facts warmed up? Are you ready to apply them to some subtraction? I hope so.

Let's make a start.

We've got two keywords in our lesson today, multiples of 10 and recombine.

So, I'll take my turn and then it'll be your turn.

So, my turn, multiples of 10.

Your turn.

My turn, recombine.

Your turn.

Well done.

I know multiples of 10 is more than one word, but it's a phrase we're going to be using a lot in this lesson and we're going to be thinking about recombining too when we're working out the answers to our subtractions.

So, look out for those and make sure you use them as you're talking about your work today.

There are two parts to our lesson today.

In the first part, we're going to subtract multiples of 10 from two-digit numbers.

And in the second part, we're really going to focus in on those known facts that we can use to subtract multiples of 10.

And we've got Alex and Sam helping us in our lesson today.

So, Sam solves this equation on a number line.

82 subtract 30, and she's drawn her number line and put 82 onto it.

"I know 30 is 3 tens, so I can count back 3 tens." She says.

82 subtract 10 is 72, subtract another 10 is 62, and subtract another 10 is 52.

And she's subtracted 3 tens by counting back.

So, 82 subtract 30 is equal to 52.

What do you notice about the tens digits and the ones digits in the equation? She says, "The tens digits change, but the ones digits stayed the same." And you can see that 2 in the minuend is the same 2 that's in the difference.

We haven't touched the twos because we've only been subtracting a multiple of 10.

Alex thinks he can use a more efficient strategy.

Ooh, let's see what his strategy is.

He says 82 is equal to 8 tens and 2 ones.

82 is equal to 80 plus 2.

And he's shown that with the base 10 blocks.

He says, "I know that 8 subtract 3 is equal to 5.

So, 8 tens subtract 3 tens is equal to 5 tens.

or 80 subtract 30 is equal to 50.

And you can see he's crossed out three of the 10 blocks.

So, now he's left with 5 tens and 2 ones, which is equal to 52.

So, 82 subtract 30 must be equal to 52.

Let's show this in a part-part-whole model.

So, we've got 82 subtract 30.

So, first, he's going to partition 82.

And he says, "I know that 82 is equal to 80 plus 2." And there it is in a part-part-whole model.

Now he needs to subtract the tens and he knows that 8 subtract 3 is equal to 5.

So, 8 tens subtract 3 tens is equal to 5 tens.

So, he knows that he's got 5 tens left and 2 ones.

So, 82 subtract 30 is equal to 52.

We can now recombine the tens we have left with the ones.

So, we've got 5 tens and 2 ones.

52.

Let's think about this with a stem sentence.

So, first we're going to partition, then we're going to subtract the tens, then we're going to recombine.

So, 8 tens and 2 ones, minus 3 tens is equal to 5 tens and 2 ones.

And we know that that is 52.

So we've partitioned, we've subtracted the tens, and then we've recombined the tens we have left with the ones.

I wonder how we could show this on a place value chart.

It's the same equation.

Let's have a look at it on a place value chart.

So, first, we're going to do the partitioning.

So, we're partitioning the 82 into 8 tens and 2 ones, and we can see that clearly on our place value chart.

And Sam says, "I know that 82 is equal to 80 plus 2." Now, we're going to subtract the tens.

She says 8 subtract 3 is equal to 5, so, 8 tens subtract 3 tens is equal to 5 tens.

So we've got 8 tens subtract 3 tens which is equal to 5 tens.

And now we're going to recombine.

And the place value chart does that really nicely for us.

There are 5 tens and 2 ones left, which is 52.

So, 82 subtract 30 is equal to 52.

Let's use that stem sentence to pull those steps together.

8 tens and 2 ones minus 3 tens is equal to 5 tens and 2 ones.

And we know that that is 52.

So, which of these represents the equation shown? We've got 92 subtract 30.

So is it A, B, or C? Pause the video, have a go, and when you're ready, we'll get together for some feedback.

How did you get on? Did you spot that it was C? So, in C, we've got 9 tens and 2 ones shown in our base 10 blocks, and we've subtracted 3 of those tens, we've crossed them out.

In the other ones, well, in our place value chart for A, we've got 9 tens subtract 2 tens.

That's not right, is it? We had 2 ones and we were subtracting 3 tens.

And in B, we've got 93 subtract 20, so, no, that's not right either, is it? It should be 92 subtract 30.

So, C was correct.

Alex says he has shown this equation on the place value chart, but what mistake has been made? It's 87 subtract 50.

So, what can you see that he's done? Alex says, "Let's work through the equation to check." Let's partition first.

87 is 8 tens and 7 ones.

Well, we can see 8 tens and 7 ones.

Now we need to subtract the tens.

Oh, he says, "I've spotted my mistake.

I subtracted from the ones." He's done 7 ones subtract 5 rather than 8 tens subtract 5 tens.

He says I should have subtracted from the tens.

8 Tens minus 5 tens is equal to 3 tens.

Ah, he's corrected it.

8 tens subtract 5 tens is equal to 3 tens.

So, when we recombine, we can see that we've got 3 tens and 7 ones.

So, our difference is 37.

87 subtract 50 is equal to 37.

Let's use our stem sentence to pull those steps together.

8 tens and 7 ones minus 5 tens is equal to 3 tens and 7 ones, which is our 37.

Time for you to do some practise now.

So, can you use a part-part-whole model or a place value chart to solve the following equations? And then you could use base 10 blocks to check that you are right.

So, we've got two sets here and two more sets here.

So, pause the video, have a go at your practise, and when you're ready, we'll get together for some answers and some feedback.

How did you get on? So, you might have done this.

So, let's think about that 97 subtract 20.

And we're going to partition it first using a part-part-whole model.

And we've got 7 ones and 9 tens.

That just helps us to see our tens together, doesn't it? Then we're going to subtract the tens.

9 tens subtract 2 tens is equal to 7 tens, and then we're going to recombine.

So, we've got 7 tens and our 7 ones, which is 77, so, 97 subtract 20 is equal to 77.

And you could use the same strategy to solve the other equations.

You might have noticed something.

We had 97 subtract 20, and then 87 subtract 20, and then 77 subtract 20.

So, each time our minuend, the number we were starting with or the whole, was 10 less.

So our answer, too, will be 10 less.

87 subtract 20 is 67.

And 77 subtract 20 is equal to 57.

What did you spot about B? Well, this time our whole or our minuend was 63 each time, but we were subtracting 10 and then 20 and then 30, so we were subtracting 10 more each time.

So, our answer was going to be 10 less each time.

63 Subtract 10 is equal to 53.

63 Subtract 20 is equal to 43.

And 63 subtract 30 is equal to 33.

So, what might we have done for C? Well, we've might have used a place value chart.

So, again, we've partitioned our 65 into 60 and 5.

Can you see that this time our whole or our minuend is not the first number.

We've got our difference or our answer is first.

So, 65 subtract 40.

Well, then we can subtract the tens.

So, 6 tens subtract 4 tens is equal to 2 tens.

And then we can recombine.

2 Tens and 5 ones is equal to 25.

So, 25 is equal to 65 subtract 40.

And you could use the same strategy to solve the other equations, but, again, you might have spotted a pattern.

65 Subtract 40 is equal to 25, and this time, our whole gets 10 bigger each time so, our difference will be 10 bigger as well.

So, 35 is equal to 75 subtract 40, and 45 is equal to 85 subtract 40.

What do you notice this time? Well, this time our whole or our minuend is the same each time.

67 Subtract 20, 67 subtract 30, and then something is equal to 67 subtract 40.

So, we could use our partition, subtract the tens and recombine, and we would find that our answers were 47, 37, and 27.

We were taking away 10 more each time, so we had 10 fewer in our difference.

And on into the second part of our lesson.

We're going to be using known facts to subtract multiples of 10.

So, let's use our number facts to find the missing numbers.

Let's start with 95 subtract 20.

How do we know it's that? Well, 95 is our whole and 20 is a part, and we know that to find the missing part, we subtract the known part from the whole.

So, 95 subtract 20 is going to be equal to our missing part.

So, I must subtract 2 tens from 9 tens.

We know that 95 is nine tens and 5 ones.

So, we're going to use the fact 9 subtract 2 is equal to 7.

So, 9 tens subtract 2 tens is equal to 7 tens.

Let's fill in our stem sentence.

9 tens and 5 ones, that's our 95, minus 2 tens, that's our 20, is equal to 7 tens and 5 ones, and that's 75.

So, 95 subtract 20 is equal to 75.

What about the next one? 95 subtract 30.

We're subtracting the known part to find our missing part.

So, I must subtract 3 tens from 9 tens.

So, the fact I'm going to use is 9 subtract 3 is equal to 6.

So, 9 tens subtract 3 tens is equal to 6 tens.

Let's fill in our stem sentence.

We're going to partition first, then subtract the tens, and then recombine.

So, 9 tens and 5 ones minus 3 tens is equal to 6 tens and 5 ones.

95 Subtract 30 is equal to 65.

Time to check your understanding.

Can you match each part-part-whole model to the fact that you would use to solve it? Pause the video, have a go, and when you're ready, we'll get together for some feedback.

How did you get on? So, did you spot that our first part-part-whole model 87 subtract something is equal to 50.

87 as our whole and 50 is a part.

So, we've got 8 tens subtract 5 tens, so, we can use 8 subtract 5 is equal to 3 to work out that 80 subtract 50 is equal to 30.

For the middle one, we had a whole of 78 and a part of 50.

So, we need to subtract the part 78 subtract 50.

So, we've got 7 tens subtract 5 tens, so we can use 7 subtract 5 is equal to 2.

And we know that 70 subtract 50 is equal to 20, so, there'll be 2 tens in our missing part.

And our last one then uses that first set of equations.

Our whole is 85 and our part is 70, so, we need to subtract 70 from 85.

So, we've got 8 tens subtract 7 tens.

So, 8 subtract 7 is equal to 1.

80 Subtract 70 is equal to 10.

So, our missing part will have a tens digit of one.

Now, let's find the number facts to help us to solve these.

Again, we've got a known whole and a known part, so we need to subtract the known part from the whole to find the missing part.

Let's start with this one.

We've got 46 subtract 20.

So, I must subtract 2 tens from 4 tens.

I will use the fact 4 subtract 2 is equal to 2.

So, 4 tens subtract 2 tens is equal to 2 tens.

Let's complete our stem sentence.

Remember we're going to partition our two-digit number, that's our whole, and then we're going to subtract the tens, and then we're going to recombine.

So, 4 tens and 6 ones minus 2 tens is equal to 2 tens and 6 ones.

46 Subtract 20 is equal to 26.

Let's look at this one.

Do you notice something? We've got 56 subtract 20 this time.

So, this time, we've got to take 2 tens away from 5 tens.

So, we can use the known fact 5 subtract 2 is equal to 10.

So, 5 tens subtract 2 tens will be equal to 3 tens.

So, let's think about our stem sentence.

partitioning our 56, subtracting the tens, and then recombining to find our missing part.

5 Tens and 6 ones minus 2 tens is equal to 3 tens and 6 ones.

56 Minus 20 is equal to 36, so our missing part will be 36.

Time to check your understanding again.

Which known fact will you use to solve the following? So, we've got a whole of 68 and a known part of 40.

So, we're going to subtract the known part from the whole to find our missing part.

Pause the video, have a look, and then when you're ready, we'll come back for some feedback.

How did you get on? Did you spot that it was B? We had 68 subtract 40, so we must subtract 4 tens from 6 tens.

We can use the fact 6 subtract 4 is equal to 2.

So, 6 tens subtract 4 tens is equal to 2 tens.

Let's complete that stem sentence we've been using, thinking about partitioning, subtracting the tens, and then recombining to find our difference or our missing part.

So, 6 tens and 8 ones minus 4 tens is equal to 2 tens and 8 ones.

So, our missing part is 28.

68 Subtract 40 is equal to 28.

Time for you to do some practise now.

So, in Question 1, you're going to use known facts to find and complete the following.

And then you're going to write down the fact that you used.

So, we've got some missing parts there, so you need to subtract the known part from the whole to find the missing part.

And in Question 2, you're going to work systematically to find all the different possible ways to complete this equation correctly.

We've got 97 subtract something, with a zero in the ones, equals something 7.

So, pause the video, have a go at 1 and 2, and when you're ready, we'll get back together for some feedback.

How did you get on? So, for Question 1, we were finding that missing part by subtracting the part we knew from the whole.

So, in the first one, we had 76 subtract 40, so we could do 7 subtract 4 is equal to 3.

7 Tens subtract 4 tens is equal to 3 tens, and then recombine with our 6.

So, 76 subtract 40 is equal to 36.

What about the next one? We had 86 and we were subtracting 20.

So, we have 8 tens and 6 ones, and we need to subtract 2 tens.

So, 8 subtract 2 is equal to 6.

8 Tens subtract 2 tens is equal to 6 tens.

So, our answer is 66.

And for the last one, our whole was 68 and the part we were subtracting was 30.

So, we had 6 tens and 8 ones subtract 3 tens.

So, we needed 6 subtract 3 is equal to 3.

6 Tens subtract 3 tens is equal to 3 tens.

So, we had 3 tens and 8 ones, so our missing part was 38.

So, in Question 2, the tens digits subtracted from 9 tens could have been any single digit from 1 to 8.

Did you spot that we had a 7 in our whole, in our minuend, and we had a 7 in our difference, in that part.

And we were subtracting a multiple of 10.

So, we could subtract any multiple of 10 right the way from 10 up to 80.

So, let's have a look at those possible answers.

So, we could have had 97 subtract 10 is equal to 87.

97 Subtract 20 is equal to 77.

97 Subtract 30 is equal to 67.

97 Subtract 40 is equal to 57.

97 Subtract 50 is equal to 47.

97 Subtract 60 is equal to 37.

97 Subtract 70 is equal to 27.

And 97 subtract 80 is equal to 17.

You could do 97 subtract 90, but it wouldn't give us a two-digit number answer.

We'd just have a one-digit answer of 7.

I hope you worked systematically and were able to find all of those different solutions to Question 2.

And we've come to the end of our lesson.

So, we've been subtracting multiples of 10 from two-digit numbers.

What have we learnt about today? Well, we've learnt that when subtracting multiples of 10 from a two-digit number, the tens digit changes, but the ones digit stays the same.

We've also learned that we can use number facts to help us subtract multiples of 10 from a two-digit number.

And did you also spot those three steps that we went through? That we partitioned the two-digit number with tens and ones, then we subtracted the multiple of 10 and using our known fact, and then we recombined the tens that were left with the ones from our two-digit number, and that gave us our answer, our difference or our missing part.

Thank you for all your hard work today, and I look forward to working with you again soon.

Bye-bye.