Lesson video

Hello, everyone and welcome to maths of Mr. Dobrowolski.

Today, we'll be looking at using the Round and Adjust strategy for our addition.

So first up on the lesson agenda, we have round and adjust when adding nine.

Then we'll complete our talk task.

Then we'll have rounded adjust when adding two digit numbers, followed by your independent task.

For this lesson, you will need a pencil and a notebook.

If you don't have these items, pause the video now and go get them.

Super.

Let's get started.

First, let's count in 10s, starting from some different numbers.

So start counting in 10s from nine and if you need some help, you can use this number line on the screen to help you.

So you can start with me, 9, 19, 29, 39, 49, 59, 69, 79, 89, 99.

Good, let's continue on.

Starting from 14, let's count in 10s.

14, 24, 34, 44, 54, 64, 74, 84, 94.

Good make sure you're counting with me, Super.

So let's start from 22, and count by 10s.

22, 32, 42, 52, 62, 72, 82, 92, and last but not least, we'll start counting at 33 and count 10s.

So Count with me 33, 43, 53, 63, 73, 83, 93.

Super job everyone.

As you can see, counting in 10s is really easy.

And that's because, only the digit in the 10s place will change, while the digit in the ones place will stay the same.

So it's super easy to count by 10.

So, I have an addition problem here, I have 24 plus nine.

How can I use my knowledge of 10, to help me add nine? I'm going to use the strategy of round and adjust.

So there's two steps here, I round and then I adjust.

So I'm first going to round.

I know that nine is close to 10.

So we'll round nine to 10.

So now we have 24 plus 10, which is something we can do mentally really easily.

So if I know 20 plus 10 is equal to 30, 24 plus 10 is equal to 34.

Well done.

Now remember, I added 10, which is a little bit unfair cause I was only supposed to add nine, so I added a bit too much.

This is where the adjust part comes in.

I need to adjust and I need to subtract one as nine is one less than 10.

So if I know 34 minus one is equal, so I need to subtract from 34.

I need to do 34 minus one.

Well, if I know four minus one is equal to three, then 34 minus one is equal to 33.

And that's our Rounded Adjust strategy.

Let's try another example.

So now we have 34 plus nine.

Remember when we round and adjust it's two steps.

First we round, then we adjust.

So first, let's round.

I know that nine is close to 10.

So I'm going to round nine to 10.

So now I have 34 plus 10, which I can do mentally and I think you can too.

34 plus 10, well 30 plus 10 is equal to 40, so 34 plus 10 must be equal to 44, excellent.

But remember we've added just a bit too much.

So if we added one more than nine, now we need to subtract, because nine is one less than 10.

We need to subtract 1.

44 minus one.

okay, four minus one is equal to three, so I know that 44 minus one is equal to 43, super.

I think we can do this one together, because you are all so good at this.

I have the equation 44 plus nine.

Again, let's use the Round and Adjust strategy.

So what can we round? Tell your screen.

That's right, we can round nine to 10.

44 plus 10, we can do mentally really easily.

Okay 44 plus 10.

I know 40 plus 10 is equal to 50, so 44 plus 10 is equal to 54, super job.

Now remember we need to still adjust, because we added just a bit too much when we round it to 10.

So now we have to subtract one.

So 54 minus one.

Okay, we know four minus one is equal to three, so 54 minus one is equal to 53, great job.

I'm always so shocked when it's time for the talk task.

So for this talk task, I'd like you to match the equation to the number line.

Four equations, four number lines.

So as usual, I will complete the first example and then you'll be off on your own.

So let's look at the Say this box.

So I have the equation 15 plus nine.

I will round nine and add 10 to 15, which is equal to 15 plus 10 is equal to 25.

Now I need to adjust by subtracting one from 25, 25 minus one, five minus one is equal to four, so 25 minus one is equal to 24.

Now which number line matches? Where did I add 10 and subtract one and then get to 24? Here we go, 15 plus 10 is equal to 25, minus one is equal to 24.

So that math just saw number line B match it.

Your turn, pause the video, complete the talk task, and I'll see you when you're ready for the answers.

Good luck.

Super job everyone.

Let's have a look at the answers.

So 16 plus nine was equal to 25, that matched.

C or number line C, 27 plus nine is equal to 36, that match number line A and 37 plus nine was equal to 46, which match number line D.

Good job, everyone.

Okay, now let's look at using round and adjust when adding two digit numbers.

So let's look at the equation that we had first.

So remember when we had 24 plus nine, we rounded nine to 10.

That's what we rounded and then adjusted by subtracting one to get our answer.

So you rounded nine to 10 and then subtracted one.

Do you think we could use that Round and Adjust strategy here? Can I add 10? Can I round to 10, and then subtract one? Well, I have the equation 24 plus 19.

And I can't really round to 10, because I'm adding 19, not nine.

So I need to round to the nearest multiple of 10.

So 19, what multiple of 10 is that closest to? I know, 19 is only one less than 20.

So I'm going to round 19 to 20.

So that means I need to add two 10s and then minus one when I adjust.

So 24 plus 20.

Well, 24 plus 10 is equal to 34 and 34 plus 10 is equal to 44.

Now we need to adjust, we need to subtract 44 minus one.

I know four minus one is equal to three, so 44 minus one is equal to 43, super.

Let's try another one.

24 plus 29.

How can you use your knowledge of multiples of 10? How can we round this? I know 29 is only one less than 30.

So I can round 29 to 30.

So now we're going to add 24 plus 30.

So we need to add three 10s, and then when we adjust, we subtract one.

So 24 plus 30.

While 24 plus 10 is equal to 34, 34 plus 10 is equal to 44 and 44 plus 10 is equal to 54.

Now we can't forget to adjust 54 minus one.

I know four minus one is equal to three.

So 54 minus one is equal to 53, super.

Last but not least, 36 plus 19.

What multiple of 10 can I round 19 to? That's right.

19 is only one less than 20.

So we can round 19 to 20.

So I can add now 36 plus 20 mentally, right and then we can't forget to adjust by subtracting one.

So first, let's add 36 plus 20.

36 plus 10 is equal to 46, 46 plus 10 is equal to 56 and now let's adjust 56 minus one, well six minus one is equal to five, so 56 minus one is equal to 55, super.

And again, it's already time for your independent task, how exciting.

So for this independent task, you're going to use the Round and Adjust strategy to solve the following equations.

You can use the number lines to demonstrate your mental calculations.

So find as much of this mentally in your head as possible.

So of course, I will do the first one and then you're off on your own.

23 plus 29, I know I can round 29 to 30 because 29 is only one less than 30.

So now I have the equation 23 plus 30, and I can't forget to adjust by subtracting one.

So 23 plus 30, well I know that 23 plus 10 is equal to 33, 33 plus 10 is equal to 43 and 43 plus 10 is equal to 53.

And I can't forget to adjust 53 minus one is equal to 52, super.

Your turn, complete the equations, use the number line if you need help.

Pause the video now and I'll see you when you're ready for the answers.

Good luck.

Super job, everyone.

So let's just go over this really quickly.

64 plus 19 is equal to 83.

You should have rounded 19 to 20.

34 plus 29 is equal to 63 and you should have rounded 29 to 30.

16 plus 19 is equal to 35 and you should have rounded 19 to 20 on that one, good.

If you'd like to, you can share your work with Oak National by asking your parent or carer to share your work on Instagram, Facebook or Twitter tagging @Oak National and #LearnwithOak.

And as always, don't forget to complete the final quiz.

It was so great to see all of you and I hope to see you in the future.

Bye.