Lesson video

Hello there, my name is Mr. Goldie, and welcome to today's maths lesson.

And here is the lesson outcome.

I can identify knowns and unknowns in addition equations.

And here are our keywords.

So I'm going to say each keyword, can you repeat the keyword back? So the first keyword is addend.

The next keyword is sum, and the next keyword is difference.

So it's important to understand what those words mean, so let's have a look at their meaning.

So an addend is a number added to another.

The sum is the total when numbers are added together.

And the difference is the result after subtracting one number from another.

So in the equation seven subtract three equals four, four is the difference, it's the result after subtracting three from seven.

And here is our lesson outline.

So in the first part of the lesson, we're going to be finding missing addends.

In the second part of the lesson, we're going to be thinking about whether we subtract or we add on.

Let's get started.

In this lesson, you'll meet Lucas and Izzy and they're going to be asking you some questions and helping you with your maths today.

Lucas and Izzy are using bar models to work out missing numbers.

We've got a bar model with sum and addend and addend on it.

And Lucas says the sum is the whole and the addends are parts.

So that's important to remember.

The sum is the whole and the addends are parts.

Addend plus addend equals the sum.

Here's another bar model with minuend, subtrahend, and difference on it.

"The minuend is the whole and the subtrahend and difference are both parts," says Izzy.

And if we take the minuend and subtract the subtrahend, we end up with a difference.

Minuend subtract subtrahend equals difference.

Lucas and Izzy take part in a birdwatch and you can see a table there with the day of the week and the number of birds that have been seen and Lucas and Izzy have been tallying the number of birds.

We counted 22 birds on Monday and 20 birds on Tuesday.

Tallying is a really quick way of counting something.

So every time you see something, you make a little mark, a little line.

And after you've spotted the fifth thing or you've counted the fifth thing, you put a line through them to make it much easier to counter at the end.

So every time you see a little group of lines with a line through it, that means means five.

So you can see on Monday that Lucas and Izzy counted 5, 10, 15, 20, 21, 22 birds.

How many birds have they seen so far? I can find the missing sum by adding the parts together.

So if you're trying to work out how many birds they saw on Monday and Tuesday altogether, add the parts together.

So sum is made up of the addend and the other addend.

Izzy calculates how many birds were seen on Monday and Tuesday.

"I have to add 22 and 22 together," says Izzy.

So addend plus addend equals the sum.

So Izzy uses the bar model to help work out the answer, and here's 22 and 20 on the bar model.

Remember they're both parts, they're the parts we're adding together to find the sum, to find the whole.

22 add 20 is actually quite an easy calculation to do, isn't it? 22 add 10, add 10 more, the answer would be 42.

So Izzy has found the whole, that's how many birds were seen altogether on Monday and Tuesday.

So finding a missing sum can be pretty easy, can be pretty tricky as well depending on the numbers, but often it's quite easy to work out what you have to do to get the answer.

Lucas counts 34 birds altogether on Wednesday and Thursday.

You might have spotted a slight problem already.

I know I saw 10 birds on Thursday, but I've forgotten the total for Wednesday.

So Lucas can remember how many birds he saw on Thursday, saw 10 birds, and he knows all together on Wednesday and Thursday he saw 34 birds, but he doesn't know exactly how many he saw on Wednesday.

"That's okay, Jacob," says, Izzy, "We can work it out." Izzy and Lucas try to calculate the answer.

"We know the whole and one of the parts," says Lucas.

So the whole is 34.

That's the number of birds that were seen on Wednesday and Thursday altogether.

And Lucas also knows he saw 10 birds on the Thursday.

Izzy and Jacob want to find the missing addend.

They want to find the missing part.

The two addends add together to make the sum 10 and this mystery number add together to make 34.

10 add something equals 34.

To find a missing part, subtract the other part from the whole.

And this is really important.

This is something you've got to remember.

So to find a missing part, subtract the other part from the whole.

So to work out what the missing part is, to work out how many birds were seen on Wednesday, we'd have to calculate 34 subtract 10.

If we subtract one part from the whole, we get the other part.

The answer would be 24.

Again, it's quite an easy calculation to do, isn't it? So 24 is the missing number.

Lucas completes the tally chart.

So I counted 24 birds on Wednesday.

So he puts in there a tally showing 24.

"I wonder how many we'll see on Friday," says Izzy." On Thursday and Friday, Lucas counts 45 birds.

Ah.

Again, you might have spotted the mistake that Lucas has made.

Oops, I've added them together again.

So Lucas has somehow added them all together again.

He doesn't remember exactly how many birds he saw on Friday.

"Not a problem," says Izzy.

"Relax, chill.

We can work out how many we saw on Friday." I wonder if you could have a think.

How would he work it out? How could Izzy and Lucas work out how many birds were seen on Friday? Calculate the answer.

We know the whole and one of the parts.

Know the whole is 45, know one of the parts is 10.

The two addends add together to make the sum.

And Izzy is just reminding us again to find a missing part, subtract the other part from the whole.

So what calculation would Izzy and Lucas have to do to work out the missing number? Pause the video, work out the calculation and work out the answer.

And welcome back.

Did you manage to work out the calculation? Did you also manage to calculate the answer? Let's have a look to see if you were right.

So the calculation you should have done was 45 subtract 10 and 45 subtract 10 is quite a straightforward calculation.

The answer would be 35.

The missing number is 35, so well done if you worked out the calculation and the answer.

Lucas completes a tally chart for Friday.

"I counted 35 birds on Friday," says Lucas and very proudly he puts in the tally marks for Friday.

Well done, Lucas.

Izzy wants to work out the missing number in an addition equation.

We got a number, add seven equals 28.

"To find the missing part, subtract the other part from the whole," says Izzy.

So we've got the bar model representing the problem as well.

28 is the whole, 28 is the sum, and seven is one of the parts.

If we start with 28, start with the whole and subtract seven, we get the missing part.

So what is 28 subtract seven? We can represent this using base 10 blocks, so here's 28 represented in base 10 blocks and we can subtract seven.

What are we left with? The answer is 21.

28 subtract seven equals 21.

21 add seven equals 28.

We can also fill in the bar model too.

Izzy has another go at a problem.

So Izzy wants to work out the missing number in an addition equation.

This time, we've got the addition equation 12 add something equals 42.

So remember to find a missing part, subtract the other part from the whole.

So again, Izzy has represented this problem using a bar model, so 42 and 12.

And to work out the answer, Izzy could change it around into a subtraction calculation.

So 42 subtract 12.

What is 42 subtract 12? Again, we can represent this using base 10 blocks.

Here's 42, 42 subtract 12 equals 30, which means a 12 add 30 equals 42.

Work out the missing number.

We've got there the calculation something add 30 equals 53.

What is the missing number? Remember to find a missing part, subtract the other part from the whole.

Here's a bar model representation to help you.

Here's the calculation turned around into a subtraction calculation, so 53 subtract 13, and here are some base 10 blocks.

We've got lots of tools there to help you work out the answer.

Pause the video, what is the missing number? And welcome back.

Did you manage to answer the question? Let's have a look to see if you were right.

So we've got there something add 13 equals 53 and we change that around into 53 subtract 13 because remember if there's a missing part, subtract the other part from the whole.

Got our base 10 blocks representing 53, let's subtract 13 and that leaves us with 40.

53 subtract 13 equals 40.

40 add 13 equals 53.

Okay, very well done if you've got 40 as the missing number.

Let's look at task A.

So in task A, you're going to work out the missing addends.

Use bar models and base 10 blocks to help you.

So you've got some bar models there, you've got some equations with missing numbers as well.

And the equation has been written as an addition and as a subtraction as well.

That's the same missing number in both of them.

So work out the missing addends.

And part two, work out the missing addends.

Use bar models and base 10 blocks to help you.

This one's slightly trickier because you're just given the calculation with the missing number in it.

You've got to complete the bar model using that equation and you've also got to change it into a subtraction calculation to help you easily calculate the answer.

Here is part three of task A.

So complete the table showing how many birds were counted each day.

Use the clues below to help you.

So the first clue you've got there is A, the total for Thursday and Friday was 17.

And you can see how many birds were counted on Thursday, how many were counted on Friday, so you know the whole, you know one of the parts, can you work out the other part? So there are three parts to task A.

Pause the video and have a go at task, A.

Welcome back.

And let's take a look at those answers and see how you got got on.

Here are the answers for part one of task A, so work count the missing addends.

So our first calculation 31 subtract 10 equals 21.

So the missing number was 21.

So well done if you completed part one of task A.

Here are the answers for part two of task A.

So our first calculation there, seven add something equals 29.

You could have changed that around into 29 subtract seven equals 22.

So the missing part was 22.

So well done if you completed part two and let's move on to part three.

In part three, we had to complete the table.

And you can see there the tally marks have also been put in as well.

So to work out Friday, you'd have to do seven add something equals 17.

So you can change that around into a subtraction calculation, 17 subtract seven equals 10.

So the number of birds counted on Friday was 10.

Very well done if you've got the answer to part three and excellent work if you completed part three of task A.

Absolutely brilliant.

And let's move on to part two of the lesson.

So part two of the lesson is called subtract or add on.

Izzy looks at more missing number problems. So here are the three calculations that Izzy is going to look at and each of them has a missing number.

So Izzy says, "To find a missing part, subtract the other part from the whole." That's what she told us earlier on, didn't she, in the first part of the lesson? Lucas says, "Do you always subtract to find a missing number in an addition equation?" What do you think? Should Izzy use subtraction to find all of the missing numbers? Izzy starts with the first problem.

So the first problem is 37 add something equals 40.

So Izzy represents that problem using a bar model.

So 40 is the whole and 37 is one of the parts.

"I could rearrange this into a subtraction equation," says Izzy.

"What is 40 subtract 37?" Lucas says, "37 and 40 are quite close to each other.

Wouldn't it be easier to add on from 37 to 40?" So Lucas knows that seven add three equals 10, so 37 add three equals 40.

And Lucas is saying that's much easier than trying to calculate 40 subtract 37.

The missing number is three.

When the numbers are close together, add on to find the difference.

So normally we subtract to find the difference, but you can also add on to find the difference as well.

Izzy looks at the next problem.

This problem is something add five equals 300.

Izzy represents this using a bar model, so 300 is the whole and five is one of the parts.

"I could rearrange this into a subtraction equation.

What is 300 subtract five?" says Izzy Lucas says, "I think that's a good idea." So 300 subtract five.

So 100 subtract five equals 95.

300 subtract five equals 295.

So the missing number is 295.

So when the numbers are far apart, subtract the known part from the whole.

Izzy looks at the last problem.

So something add 360 equals 370.

Izzy represents this problem using a bar model with 370 representing the whole and 360 representing one of the parts.

"I could rearrange this into a subtraction equation," says Izzy.

Izzy does like subtraction, doesn't she? What is 370 subtract 360? Lucas says, "Well, you could subtract, but wouldn't it be easier to add on from 360 to 370? Wouldn't it be easier to add on to find the difference?" So Lucas says, "I know that 60 add 10 equals 70, so 360 add 10 equals 370." So the missing number is 10.

So when the numbers are closed together, add on to find the difference.

Izzy looks at a different problem, something add 40 equals 70.

Izzy represents the problem using a bar model, so 70 being the whole, 40 being one of the parts.

"I could rearrange this into a subtraction equation," says Izzy.

What is 70 subtract 40? "You could add on to find the difference," says Lucas.

40 add something equals 70.

What do you think? Which method would be easier? 70 subtract 40 equals 30 and 40 add 30 equals 70.

Which would be easier? Either method works.

70 and 40 are not particularly close together.

They're not particularly far apart.

So when you've got numbers like that, it doesn't really matter which method you do, it's whatever one is easier for you and sometimes depends on the numbers that you are calculating with.

You might spot that it's easy to add on to find the difference or it might be easier to subtract to find the difference.

How would you solve these problems? So two problems there.

Three add something equals 100 and something add 98 equals 100.

"Would you subtract?" says Izzy.

Would you add on to find the difference? Pause the video and have a go at trying to find those missing numbers.

And welcome back.

Did you manage to find the missing numbers? Let's have a look to see if you were right.

So let's look at that first problem.

So where the numbers are far apart, subtract the known part from the whole.

Three and 100, there's a big difference between those two, isn't there? So it's much easier to subtract the smaller part from the whole.

So you could do the calculation 100 subtract three or 100 subtract three equals 97.

So the missing number, the missing part was 97.

Let's look at the second problem.

So when the numbers are close together, add on to find the difference.

98 and 100 are quite close together.

You don't have to add very much to 98 to get to 100.

98 add what equals 100? This number is two.

The two add 98 equals 100.

Very well done if you calculated both missing numbers and very, very well done indeed if you thought really carefully about whether you use subtraction or adding on to find the difference.

Izzy has these missing number equations.

She's got four missing number equations there.

"I'm going to work out the best way to solve them," says Izzy, so she's going to arrange them onto this table.

So on one side, we've got subtract.

On the other side, we've got add on to find the difference.

And Izzy's going to think very carefully about which strategy, which method she's going to use to help her work out the answer.

Izzy says, "To find this missing number here, it'd be easier to add on to find the difference." Something add 60 equals 65 60 and 65 includes together.

To find that missing part, it'd be much easier to add on to find the difference.

For this equation here, the numbers are far apart, so something add 15 equals 115, there's a big difference between 15 and 115.

It'd be much easier to subtract to find the difference.

And in fact, you might spot something about those two numbers, 115 and 15.

I think it would be pretty easy to subtract 15 from 115 as well.

20 add something equals 230, numbers are far apart, so it'd be much easier to subtract 20 from 230.

And our last one, 89 and 91 are close together.

To find the missing part, it'd be much easier to start on 89 and add on to get to 91.

Can you help Izzy find the missing numbers? So Izzy says, "Remember to subtract or add on to find the answer." So there are four calculations there, four missing numbers to find.

Think carefully about how you're gonna work out the missing number.

Pause the video and see if you can find those four missing numbers.

And welcome back.

Let's take a look and see whether you were right.

That first calculation, something add 15 equals 115, Izzy thinks we should use subtraction to work out the answer.

So you could turn that calculation around and do 115 subtract 15.

And that's quite an easy calculation to do.

115 subtract the 15 equals 100.

So the missing number there is 100.

To work out the missing number in 20 add something equals 230, again we can use subtraction, start with 230, subtract 20 and the answer would be 210.

And then we've got two calculations where we have to add on to find the difference.

What do you have to add to 60 to get to 65? That's quite an easy calculation, isn't it? So the answer would be five, the missing number is five, and a slightly trickier one because you have to cross a tens number.

What do you have to add on to 89 to get to 91? So 89, you add on one to get to 90, what do you have to add on to get to 91? You'd have to add on two.

89 add two equals 91.

Very well done if you answered all four of those questions correctly.

And let's move on to task B.

In task B, we're gonna be thinking about how would you solve the missing number problems? Would you subtract to find the answer or would you add on to find the difference? Also, can you answer the questions as well? And you might want to do that as you go along.

And as you sort the calculations, have a go trying to answer them as well.

So here are the 15 calculations that you are going to be sorting.

You're gonna cut them up, put them on the table, work out where you're gonna put them, think about whether you're going to subtract or add on and try to calculate the answers as well.

So pause the video and have a go at task B.

And welcome back.

How did you get on? Did you manage to sort all of the calculations? Did you manage to answer them all as well? If you did, excellent work, but let's see whether you got them right.

So this is how Izzy sorted them.

So she's put down one side of the calculation, she would've used subtraction to work out the missing number.

And on the other side, she put the calculations where she would use adding on to find the difference.

So 35 add three equals 38.

We could have turned that around into the calculation 38 subtract three equals 35.

For seven add 160 equals 167, we could have turned that around into the calculation 167 subtract seven equals 160.

On the other side, we've got the ones where we're adding on to find the difference.

So 600 add 100 equals 700.

Very well done if you thought carefully about how to sort those calculations.

Don't worry if not every single one is exactly the same place as Izzy put them as long as you've been thinking really hard about how to calculate each missing number efficiently and quickly.

So excellent work today, and I hope you're feeling much more confident about finding knowns and unknowns in addition calculations.

And let's move on to our lesson summary.

So use subtraction to find a missing part in an addition equation.

When the numbers are far apart, subtract the known part from the whole.

When the numbers are close together, add on to find the difference.