Lesson video

Hi, everyone, my name is Miss Jeremy.

Thank you for joining me for another math lesson.

Today's math lesson is focused on adding and subtracting using the Round and Adjust strategy.

So find yourself a nice quiet space and get yourself ready for learning and then press play to begin the lesson.

Let's begin by looking at our lesson agenda.

We're going to begin with a warm up where we'll look at derived facts.

We're then going to look at Rounding and Adjusting for subtraction.

Before looking at Rounding and Adjusting for addition, We'll end by looking at how we can apply our understanding of the Round and Adjust strategy before your independent task and quiz.

For today's lesson, you'll need a pencil and some paper and a nice quiet space.

Feel free to pause the lesson here and get your resources sorted and then restart when you're ready.

Let's start with our warmup where we'll be looking at some derived facts.

So reminding ourselves derived facts are taken from known facts, we look at the information that we know and we see what we can work out from that information, we see what we can derive from that information.

So I've got a known fact on the screen here, we know that 21, subtract eight is equal to 13.

So what other facts can we derive from this? I'm going to give you one and then I'm going to ask whether you can give me a few as well.

So the first thing I know is that if 21 minus eight is equal to 13, what if I were to multiply every single digit, every single number in that equation by 10, I would be able to derive that 210 minus 80 is equal to 130.

So what I've done is I've taken every digit in that equation, every number in that equation, I've multiplied it by 10.

And I've created a derived fact.

So now can you do the same thing can you create two more derived facts from our known fact, I'm going to give you five seconds.

Okay, so in the same way that I multiply by 10, we could multiply a known fact by 100.

So we can actually derive that to 2100 minus 800 is equal to 1300.

But all I've done is I've multiplied all of the numbers in our known fact by 100.

To create our derived fact, in the same way, we could multiply all of our numbers in our known fact by 1000.

So we'd have 21,000 minus 8000 is equal to 13,000.

Really simple three derive facts that from one known fact, we could continue to do this we could multiply by 10,000, we can multiply by 100,000.

And we could derive as much many factors as we wanted from that one known fact.

So you can see here that actually derive facts are really useful when it comes to larger equations, we can help ourselves with answering those larger equations using those known facts that we already know.

So moving on to another strategy today, we've learned about how to use derive facts, but I want us to look at this new strategy called Round and Adjust.

And we're going to start by looking at Rounding and Adjust strategy in the context of subtraction.

So look at the equation that's on the screen, it says 8300 subtract 999.

And our question is there a method other than the column method to solve this? So what I could do if I wanted to was I could write out 8300 minus 999.

And I could use my column method to solve that.

Two reasons why this isn't necessarily the best approach.

Firstly, that's going to take me quite a while I'll need a piece of paper I will need a pencil.

It's going to take me quite a while to do that.

Secondly, it involves regrouping in three columns.

So I can already see I'm going to have to do a lot of regrouping.

To answer that question.

I think there's an easy strategy.

And I think that strategy is the Round and Adjust strategy.

Let me show you how it works.

Well, I know that the number 999 is very very close to 1000.

What if I were to round up to 1000 and then adjust for it later on.

Let me show you how that would work.

So because I'm subtracting, I'm going to start with my larger number up here 8300.

Now, I could try and take away 999 from that but actually, what if I were to do a really big jump and just take away 1000.

So here, I'm going to jump all the way back here.

I'm going to subtract 1000 from that number, because that's super easy to do.

The only digit changes is the digit in the thousands column.

So the number would be 7003, just here.

Now I have subtracted 1000.

But that is one too many, I only wanted to subtract 999 and I subtracted 1000.

That is one too many.

So because I subtracted one too many, I'm going to have to add on an extra one.

So I'm going to add on one to my answer here.

I'll show you using that little jump there.

So my answer is 7301, 7301.

Let me talk you through those steps again.

So I started with my larger number 8300.

Because 999 is so close to 1000, I subtracted 1000.

But because that was one too many, and I only needed to subtract 999.

I added one back on, my final answer was 7301.

Let's look at another example.

Have a look this one here, I've got the equation 4500.

This time subtracting 998.

I'd like you to draw me a number line.

Use the Round and Adjust strategy to subtract 1000 and then add on whatever you need to add on.

What is the answer to this question? Use the sentence starters that I've provided for you there as well to help you with explaining what you're doing as you're doing it.

Pause the video now to complete your task and then resume it once you're finished.

Okay, let's go through the answers.

Let's see what we get.

So once again, I'm starting with my larger number.

I've got 4500 here.

Now I'm subtracting 998.

That is very, very close to 1000.

So I'm just going to subtract 1000 to begin with.

I'm going to do a big jump all the way backwards.

And I'm subtracting 1000 and 4500 subtract 1000 is equal to 3500.

But that is two too many can you see only wanted to subtract 998? And I actually subtracted 1000.

That's two too many.

So I'm now going to need to add two to my answer.

And add on a jumper of two, 3500 plus two is equal to 3502.

So my answer is 3502.

Using this sentence starters you should have said, I have subtracted 1000 that is two too many.

I need to add two to my answer.

How did you get on with that? Let's have a look at some examples involving addition now.

So this time exactly the same concept, but we're adding so it's going the opposite way on our number line.

This equation says 3250 plus 990.

So again, 990 is very close to 1000.

And instead of adding 990 using column method to moment number, I'm going to use a number line I'm going to route to adjust.

I'm starting with 3250 here, I'm going to add 1000.

And that takes me to 4250.

I have added 1000 but that is 10 too many.

I only wanted to add 990 and I added on 1000.

That is 10 too many.

So because I added on 10 too many, I need to subtract 10 from my answer.

So I'm going to subtract 10.

And that takes me to 4240.

Remember the only digit that's changing is the digit is my tens and subtracting 10, so my answer is 4240.

So you can see we can use rounding and adjusting for subtraction.

We can also use it for addition.

Now is your can you use the round and adjust strategy to solve this one, 6329 plus 995.

Remember which way you're starting from so Place your 6329 at the beginning of your number line because you're adding, add on your thousand and see what you need to adjust for afterwards.

Pause the video now to complete your task and resume it once you're finished.

Okay, let's look at it together.

So I'm starting with 6329.

Now because 995 is so close to 1000, I'm going to add that instead, we're going to add a big jump of 1000 and that takes me that to 7329.

I have added 1000.

Now I can see that I originally wanted to add 995, I added 1000 that is five too many.

I need to now subtract five from my answer.

So I'm going to be subtracting five.

And the only digit that's going to change is the digit that's in my ones column, because I'm doing nine minus five.

So my final answer is 7324, 7324.

So let's move on adding on a little bit.

Let's see if we can apply the Round and Adjust strategy to other multiples of 10.

So look at this example here I've got 5380 plus 2950.

So this time, my number I'm adding on isn't close to 1000, but it is close to another multiple of 10, is close to 3000.

So what if I were to begin with 5380 and this time I add on 3000 ooh beg your pardon for the slightly wobbly line there.

So I'm adding on 3000, the only digit that's going to change is that digit in the thousands column, what is 5380 plus 3000, it is 8380.

Now, I have added 3000 to my number.

That is, and I can see here 50 too many, because 2950 is 50 less than 3000.

So I've added 50 too many on to my number.

So I now need to subtract 50 from my answer.

So I've got 8380 minus 50 what does that equal? Use your known facts, look at the digit that's in the tens and you're subtracting your 80 subtract 50 What is your final answer? Final answer is 8330.

So, my final answer here is 8330.

So you can see that you can also use round and adjust when it comes to numbers that are greater than 1000.

So if your number is very close to an easier number to add or subtract, use that to help you and then adjust for it later round up or down to it, and then adjust for it afterwards at the end.

Your turn to have a go at least one of these examples.

So you've got 9400 subtract 3980, I would like you to remember start at the end of your number line, because you're subtracting this time.

Can you subtract a greater number and adjust for it afterwards? Pause your video now and resume it once you're finished.

So let's have a look together.

Well, I'm going to start with 9400 on this end here.

And I can see that 3990 is very close to 4000.

So I'm going to subtract 4000 instead.

And I know that the digit that's going to be changing is this one just here.

So 9000 subtract 4000 is equal to 5000.

So I've got 5400 over here.

Now because I subtracted 4000, that was 20 too many, that was 20 too many to subtract.

So I now need to add 20 to my answer, and add on 20, to my answer, now the answer is 5420.

So the answer is 5420, brilliant stuff.

So it's now time for you to have a go at practising this independently.

You've got six questions on the board.

Some of them are addition.

Some of them are subtraction, so pay attention to that symbol.

I'd like you to use the Round and Adjust strategy to calculate the answers to these questions.

Spend a bit of time on this now.

Pause the video to complete your task and then resume it once you're finished.

How did you get on with that? As you can see on the screen here, we've got the answers to the questions.

If you'd like to spend some time pausing the video now to mark your own work, see how you got on and then restart when you're ready.

Thank you so much for joining me for today's lesson.

It's been great to have you.

If you'd like to please ask your parent or carer to share your work on Twitter, tagging a @Oaknational and #LearnwithOak.

Now it's time to complete your quiz.

Thank you for joining me for today's lesson.

It's been great to have you do join me again soon.

Bye-bye.