Lesson video

Hello everybody.

I'm Mrs. Crane, and welcome to today's lesson.

In today's lesson, we're going to be deriving addition and subtraction facts by using given calculations.

I'll explain exactly what that means in a moment.

Don't worry about getting anything together just yet, I'll explain all the equipment we need.

I hope you're having a good day and I hope you've your maths brain switched on.

If you can, please turn off any notifications on your phone, tablet or whatever device you're using to complete today's lesson.

Then, if you can, find somewhere nice and quiet in your home where we're not going to be disrupted, in today's math, math, can't speak this morning, in today's maths lesson.

When you're ready, let's begin.

Okay then, let's have a look through today's lesson agenda.

So, we're going to start off by looking at what's the same and what's different.

Then, we're going to look at let's explore, if I know, what else do I know? Then, we're going to look at four digit numbers, and then, for your independent task today, we're going to be looking at using given facts to solve a new equation.

For today's lesson, you will need a pencil and some paper.

So, please pause the video now to go and get those things, if you haven't got them already.

Okay, welcome back.

Let's get started then.

What's the same and what's different? Have a really close look at a and b that are on the screen.

What is the same and what is different? Well, I can see that the same, we've got two hundreds, we've got eight tens.

Oh, we've got different amount of ones, we've got three ones here and two ones here.

So, you've got one fewer ones in question b, and both of them are having 309 added to them.

So, we're going to think about how we can use the answer to one of the calculations to help us work out the answer to the other calculation.

So, let's see.

How could you work out the sum of b if the sum of a is 592? Remember, what we said about those two numbers? The difference between them was one, there was one fewer in 292, than there is in 200, I just said 292, didn't I? There's one fewer in 282, sorry, than there is in 283.

So, my answer was 592 here.

What's my answer going to be for b? Let's have a look at our dims, we're not going to use our dims all of the time today, but they're here just to show you, if you want to, you can draw your own dims at home.

So, take away one of our ones, this is two ones.

What will happen to my answer? My answer will also have one taken away from it and give us the new answer of 591, because we're adding a number that's one smaller than the number in this equation.

Okay.

Next question then.

Let's have a look at these two equations, on what's the same or what's different, because in a moment you're going to have a think about how you could work out b, this missing number here, based on a.

So, here we had 478 plus 89 is equal to 567, b, we've got 478, the same number, this time you don't know what's being added to that, we do know the answer, the sum is 577.

What's the same and what's different between these two numbers? Have a really close look.

If you're feeling confident, pause the video now to have a go at answering this question.

If you're not feeling so confident, don't worry, we're going to go through it now.

I've noticed that six tens is one less than seven tens.

So, this is one greater 10, has one greater 10, than this number here.

So, I think my number that I'm adding has one greater 10, because my answer has one greater 10.

So, I think this number is going to be 99.

Absolutely.

My answer is increased by 10.

So, the number that I'm adding has also got to increase by 10.

If you paused that and you worked that out yourself, really, really impressed, well done.

It's quite tricky.

We're going to go through a few more examples before you're doing anything completely independently today, okay? Before we do, we're going to look at subtracting.

So, it's a little bit different.

So, you've got to listen really carefully to the subtracting one.

So, let's see.

What's the same and what's different between a and b? Look really closely.

What can you spot? Well, I can see 736 is the same of both equations.

I can see three hundreds and eight tens are the same, but this equation a, has two ones, equation b has only got one one.

So, how are we going to use the answer to equation a, to work out equation b? Now, I can see here that I am subtracting 382 to give me 354.

Here, I'm only subtracting 381, I'm subtracting one fewer, I'm subtracting one less one than I was here.

So, because of that, my answer is going to be one more.

So, here my answer is going to be 355 because I've subtracted one less than I did in my first equation.

This time, if you're feeling really confident, I want you to have a think about what's changed in our equations and what the missing number is going to be.

If you're not feeling so confident, don't worry, we're going to go through it together now.

So, 847 subtract 239 is 608.

Now, I don't know my answer, but I do know 947.

So, what's changed here, it's increased by 100.

Subtract 239, same number here, we don't know the answer.

I do know is that I have taken away 100 less, I started with 100 more, sorry, I'm not taking away 100 less, I started with 100 more.

So, if I started with 100 more here, then I end with 100 more here.

So, what's 100 more than 608? Fantastic.

It's 708.

So, using that calculation here has helped us answer this calculation here.

Now, let's explore the if I know, then I know.

Hmm.

So, if I know 237 plus 586 is equal to 823, then I know what happens if one part is increased by one, this time.

What happened if one part is increased by 100? what happens if one part is decreased, decreased by 10? What if the whole is increased by 10? What I want you to do, in a moment, is you're going to pause your video and you're going to have a go at answering each of these questions.

What I would do, is use this equation here to help you.

So, I would increase one of those parts by one and work out the answer and see what happens to the whole.

You can use this here to help you if you want to speak out loud and practise it while doing it, but don't worry that it's up to you, okay? We'll go through this together in a moment.

So, pause the video now to have a go at today's let's explore.

Okay then, let's look at how we could answer that.

So, if one part's increased by one, let's have a look at our original number.

We've increased one of the parts by one.

So, it's now 238, it is increased to 239.

So, my answer has also increased by one.

This time, if one part's increased by 100.

Back to my original number 238, and I've increased it to 338, which means my answer has also increased by 100, from 824 to 924.

This time, what happens if one part decreases by 10? So, I've taken 238 and I've taken 10 away from it to give me 228 because I've taken 10 away and I'm adding 10 less, my answer also has to be 10 less.

So, my answer would be 814.

This time, what if my whole was increased by 10? My whole was increased by 10, from 824 to 834.

One of my parts also must be increased by 10.

So, I've increased 238 to 248 to show what would happen to one of my parts if my whole was increased 10.

So, because we've looked at that now we're going to move on to look at what's the same and what's different with four digit numbers.

So, have a look really closely at these two equations.

What is the same and what is different between them? Well, I can say, the number that I'm adding is the same on both of them.

I'm adding 1,309 in both equations.

I can see I've got two thousands.

I can see however, I've got three hundreds here and six hundreds here.

I've still got 79 in both equations.

So, I know that 300 is 300 less than 600.

So, there's a difference of 300 in my first number here.

So, see how that's going to make an impact on my answer.

So, I have the number 2,376 and I'm adding 1,309.

My answer is 3,685.

This time I'm adding 300 more because 600 is 300 more than 300.

So, my answer is also going to need to have 300 more.

This place value grid here is there to help you, if you wish to use that, I'm not going to go through my examples with it, but if you want to use it to help you when you pause the screen, please do.

So, if you want to pause the screen really quickly to work out what your answer's going to be, you can.

If you don't feel so confident, that's okay, we've talked it through together, I'm going to go through the answer together as well.

So, my answer, because my number is 300 more, it's going to be 3,985, 300 more than the first answer.

This time now, what is the same and what's different about these numbers, by looking at subtraction? Okay, I can straight away see that the wholes I'm taking away from it's the same in both, it's 8,139.

I can see that the thing that's different is my tens column here and here.

I can see I've got six tens here, I've got three tens here.

There's a difference of four tens, 40 between the number I'm subtracting.

So, let's see what will happen there.

Here I had 8,139, I'm subtracting 2,367, sorry, it's giving me the answer 500, that's not 100, 5,772.

Now here, I'm subtracting 40 less because my number is 40 less than this.

If I'm subtracting 40 less, my answer needs to be 40 more because I've subtracted, I haven't taken away 40, I have this 40 more left in my answer.

So, my answer would be 5,812.

The reason being, when I've added 40 to 70, I've needed to regroup.

So, my seven's become an eight and I've been left with one one.

So, although it looks a bit more tricky here, it's just because I've had to regroup when I've added them.

Right then, it's now time for today's independent task, using given fact to solve a new equation.

So, I'm just going to hide myself so you can see really clearly, your questions today.

So, you have six questions today.

What I'd like you to do is use the given fact, which is the fact at the top of each box to work out the addition and subtraction calculations.

My challenge to you today is can you explain how you're given fact helped you just solve your new equation? Please pause the video now to complete your task.

Don't forget to resume it once you're finished to go through the answers together.

Okay, welcome back.

Let's have a look at the answers then.

So, we're going to look here, at our answer.

So, my question was 1,453 plus 2,178 is equal to, my answer is 3,631.

The reason is it's 10 less than the number here.

So, my answer is 10 less.

Next then, 3,524 subtract 1,237.

My answer is 2,287.

The reason being I subtracted 100 less.

So, my answer has to be 100 more.

Next, 3,266 plus 3,276 is equal to 6,545.

The reason I've added 1,000 more, then my answer has to have 1000 more.

Next then, are the last three questions, 3,761 subtract something is 1,605, subtract 2,156, my answer is 200 less.

So, the number I'm taking away has to be 200 more.

Next then, something add 5,509, is equal to 8,131.

My number is 200 and, 200, 2,622.

If I look here in my answer, I know that the difference between eight and five, eight and three is five, sorry.

So, there's 50 less in my answer.

So, I need to be adding a number that's 50 less than this number here.

Then I have 4,075 plus something is equal to 6,971.

My answer is 2,896 because here the difference between four and seven is three, I know that's three tens.

So, I need to be adding three more tens to get that answer.

If you'd like to, please ask your parent or carer to share your work today on Twitter, by tagging @OakNational and using the #LearnwithOak.

Great work today at completing all of those facts.

Now, don't forget to go and complete your quiz and show off all the fantastic knowledge you've learned today.

Hopefully I'll see you again soon.

Thank you, and bye bye.