# Lesson video

In progress...

Hello, everyone.

And welcome to math with Ms. Dobrowolski.

Today, we'll be looking at using the round and adjust strategy for solving subtraction problems. So, let's take a look at the lesson agenda.

First, we'll be looking at round and adjust when subtracting nine, then we'll have our Talk Task, then we'll look at round and adjust, round and adjust, when subtracting two digit numbers, and then you'll be off for your Independent Task.

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

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

Super, so let's do a little warm-up.

We'll start by counting back in tens from the numbers on your screen.

So, we'll start by counting back in tens from 51.

If you're not too sure, all you need to do is look at the screen and just count on using the number line.

So, count with me, 51, 41, 31, 21, 11, one.

Super.

Let's try that with 74.

We're going to count back in tens.

Say it with me, 74, 64, 54, 44, 34, 24, 14, four.

Good job, everyone.

Do that with 87.

Count back in tens, 87, 77, 67, 57, 47, 37, 27, 17, seven.

And last, but not least, let's try that with 98, counting back in tens, 98, 88, 78, 68, 58, 48, 38, 28, 18, eight.

Good.

You'll notice, counting back in tens is really easy because, if you look, it's only the digit in the tens place that changes.

The digit in the ones place doesn't change when counting back by 10.

So, that's why I really like counting back in tens.

It's so easy.

Now, now that we've had a look at counting backwards in tens, let's see if that can help us with the round and adjust strategy.

So I have this equation, 44 minus nine.

How can I use my knowledge of multiples of 10 to help me solve this? Well, I'm going to use the round and adjust strategy.

I know nine is only one less than 10, so I'm going to round nine to 10.

So 44 minus 10, we can do that really easily.

So when we rounded and adjust, the first step is to round, which is what I just did.

And then step two, is going to be to adjust, which we'll do a little bit later.

So first 44 minus 10.

Okay, if I know 40 minus 10 is equal to 30, then 44 minus 10 is equal to? That's right, 34.

Okay, now here comes the adjust part.

You see, when we rounded and then subtracted 10, we went a little too far back.

We went too far back by one because 10 is greater than nine by one, so now we have to adjust and add that one back.

So 34 plus one.

Okay, I know four plus one is equal to five.

So 34 plus one is equal to? 35, well done.

Let's do another example.

Okay, 54 plus nine.

Let's round.

Okay, nine is really close to 10.

So I'm going to round nine to 10, 54 minus 10.

We can do that really easily mentally.

Okay, 54 minus 10.

I know 50 minus 10 is equal to 40, so 54 minus 10 is equal to? 44.

Okay, we add that one back, 'cause we went one too far, so we have to add it back.

44 plus one.

Okay, four plus one is equal to five, so 44 plus one is equal to 45.

Okay, so now we have 64 minus nine.

Let's use the round and adjust strategy again.

So I'm going to round nine to 10.

So now I have the equation, 64 minus 10.

Okay, 64 minus 10.

I know 60 minus 10 is equal to 50, so 64 minus 10 must be equal to? 54.

So remember, we subtracted a little bit too much.

We subtracted one too many, so we have to add one back now.

54 plus one.

I know four plus one is equal to five, so 54 plus one is equal to 55.

Super.

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

And of course, I'll do the first example before you go off on your own.

So, I have to say this box here, so you know exactly what mathematical language you need to use.

I'll start.

I have the equation 25 minus nine.

I will round nine to 10.

So I will round and subtract 10 from 25, which is equal to, hmm, 25 minus 10 is equal to 15.

Now, I need to adjust by adding one to 15, which is equal to? 15 plus one is equal to 16.

So let's see, which number line matches that.

Where did I subtract 10 from 25, add back one? Here we go.

I subtract 10 from 25, add one to 15 to get 16, so that matches number line B.

Your turn, solve the equations and match the number lines.

I'll see you when you're ready.

Good luck.

Super job, everyone.

Let's go over this super quickly.

So 26 minus nine was equal to 17 and that matched number line C.

37 minus nine is equal to 28, that matched number line A.

47 minus nine is equal to 38 and that match number line D.

Okay, let's take it a step further now.

Let's round and adjust when subtracting two-digit numbers.

So remember, when we completed 44 minus nine, we rounded to 10, so we had 44 minus 10, and then we added one back, which is equal to 35.

Okay, let's see, can I use that strategy for this equation? 54 minus 19.

Can I subtract 10 and add one? Well, that's not really going to make sense here because I'm not subtracting nine, I'm subtracting 19.

So really, I need to round 19 to the nearest multiple of 10 that I know.

So 19, what multiple of 10 is that closest to? Do you know? Oh yes, 19 is only one less than 20, so I can round 19 to 20.

So now I can subtract 54 minus 20.

Okay, 54 minus 20, so I have to subtract two tens and then I have to make sure I add one back.

Okay, so 54 minus 20.

I know 54 minus 10 is equal to 44 and then 44 minus 10 is equal to 34 and then I have to adjust, I need to add that one back.

34 plus one is equal to 35.

Excellent.

Let's try this one.

54 minus 29, hmm.

What multiple of 10 is 29 closest to? That's right, 29 is only one away from 30.

So I can round 29 to 30.

54 minus 30.

And then I have to remember, since I rounded, I'm going to have to adjust and add one back at the end.

So 54 minus 30.

Okay, I know 54 minus 10 is equal to 44, 44 minus 10 is equal to 34, and 34 minus 10 is equal to 24, and 24 plus one, because I need to add one back, 24 plus one is equal to? That's right, 25.

Excellent.

Okay, here's one more.

56 minus 19.

Can you tell me how you're going to round? What number will you round and what number will you round it to? Tell your screen.

That's really clever.

Rounding, we're going to round 19 to 20, because 19 is only one less than 20.

And don't forget, we're going to have to adjust later and add that one back.

So 56 minus 20.

Okay, 56 minus 10 is equal to 46 and then 46 minus 10 is equal to 36, and now we have to add one back.

So 36 plus one, I know six plus one is equal to seven, so 36 plus one is equal to 37.

Excellent.

So, for this Independent Task, I'd like for you to use the round and adjust strategy to solve the following equations.

Use the number line to demonstrate your mental calculations.

So, we want to be doing as much of this as we can, mentally.

So, 63 minus 19.

Well, of course, I'll do the first one as usual, 63 minus 19.

Okay, I'll round 19 to 20 and then I'll adjust by adding back one later.

So 63 minus 20, 63 minus 10 is equal to 53, 53 minus 10 is equal to 43, and now I have to add one back, 43 plus one, three plus one is equal to four, so 43 plus one is equal to 44.

See you on the other side.

Good luck.

Good job, everyone.

So let's go over this really quickly.

84 minus 19 was equal to 65 and you should have rounded 19 to 20, 64 minus 29 is equal to 35 and you should gave rounded 29 to 30.

36 minus nine is equal to 27 and you should have rounded nine to 10.