# Lesson video

In progress...

Hello everyone, welcome to maths with Ms Dobrowolski.

In today's lesson, we'll be subtracting two digit numbers involving regrouping.

Let's have a look at today's lesson agenda.

So first up, we're going to review our subtraction of 10.

Then we'll be looking at mental calculations using Dienes.

Finally you're off for your final quiz For today's lesson you will need a pencil and a notebook and possibly some pasta, some dry pasta, such as spaghetti or penne or macaroni.

Now I know that sounds funny, but if you have these items, please do go get them they will help.

If you don't have this pasta, that's completely fine.

As long as you have a pencil and notebook.

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

Great.

So let's review our subtraction of 10.

I have 13 minus 10 here.

Well, this is easy.

I know 13 is made up of one 10 and three ones.

So if I subtract 10, all I'm left with is three.

So 13 minus 10 is equal to three.

Complete all of these subtraction equations.

You can do it out loud, or you can write it in your notebook if you choose.

Pause the video now, when you're ready and you have the answers resume, and we can go over it.

Great job everyone.

So, 14 minus 10 is equal to four.

15 minus 10 is equal to five.

16 minus 10 is equal to six.

17 minus 10 is equal to seven.

18 minus 10 is equal to eight and 19 minus 10 is equal to nine.

So as you'll see in all of these equations, when we subtracted the 10, we were left with the amount of ones from each of them.

14 minus 10 was equal to four.

I only had the ones left.

Great.

So let's move on.

So here we have the builders really hard at work building houses.

Wow that's a lot of different machinery they're using.

So it looks like the builders used 31 bricks, so note, they started with 31 bricks.

So the builders started with 31 bricks and used 14.

How many bricks do they have left? Well, I can use my part whole model to help me solve this.

So let's have a look and see how I did that.

[ Ms Drobowolski ] So let's first identify the whole.

Well I know that 31 is our whole because that's the number we'll be subtracting from.

It's the amount of bricks the builders started with.

So, we're going to put 31 in our whole.

31 has three 10's, 10, 20, 30, and one one.

31.

We know one of our parts is 14 because that's what we're subtracting and the other part we don't know yet, that's what we have to figure out.

So how can we subtract 14 from 31? Well, one strategy I can think of is partitioning the second number.

I know that 14 can be partitioned into one 10 and four ones.

So first we subtract our 10's.

If I know that three 10's minus one 10 is equal to two 10's, then I know 31 minus 10 is equal to 21.

Now let's subtract our ones, but I'm going to use the Make 10 strategy to help us.

So I know that 21 minus one is equal to 20, so I can partition the four into one and three.

That's a really not so nice three, there that's better.

I'm going to partition the four into one and three.

So 20 minus one is equal to 20.

But I still have to subtract three.

Oh no, I know what I can do.

I can regroup.

So I will regroup one 10 into three one's, so I can subtract my three ones, okay.

So we regroup and we need 10 ones now, count with me.

One, two, three, four, five, six, seven, eight, nine, 10.

Excellent, so now we have 20 minus three is equal to 10, 11, 12, 13, 14, 15, 16, 17.

So our missing part was 17.

31 minus 14 is equal to 17.

So the builders have been hard at work again.

And this time they started with 31 bricks and used 15.

So how many bricks could they have left? Well again let's use our whole part model.

So our whole is 31 because that's the number we're subtracting from.

So we need three 10's, 10 20, 30, and one one.

So how can we subtract 15 from 31? Well, I'm going to partition the second number.

I have one 10 and five ones.

Let's subtract our 10's.

Three 10's minus one 10 is equal to 20.

So 31 minus 10 is equal to 21.

Excellent.

So now we still have to subtract our five.

Hmm.

Well I know 21 minus one is equal to 20.

So I can partition the five into one and four.

So 21 minus one is equal to 20, but I still have to subtract four.

Oh no, I have no one's left.

Well, what can I do here? I can regroup.

So I'm going to take one 10 and regroup for 10 ones.

Count with me.

One, two, three, four, five, six, seven, eight, nine, 10.

Excellent.

So now we have to subtract our four ones.

So I know 10 minus four is equal to six.

So 20 minus four is equal to, that's right, 16.

10, 11, 12, 13, 14, 15, 16.

So that means 16 is our missing piece.

What we really did, 31 minus 15 is equal to 16.

But when we subtracted, we had 31 minus our 10 and minus our five, which is equal to 31 minus 15.

And yet again, the builders have been hard at work.

This time they started started with 31 bricks and they used 16.

How many do they have left? 31 minus 16.

Well, first let's identify our whole.

I know 31 is the hole because that's the number we'll be subtracting from.

So we'll place 31 into the whole.

31 has three 10's.

10, 20, 30, and one one.

Excellent.

I know 16 will be one of our parts, and the other part we don't know.

That's what we have to figure out.

So how can we subtract 16 from 31? Well I know that we can partition the 16 because we can partition the second number.

That's one of our strategies.

So 16 can be partitioned into one 10 and six ones.

First we subtract our 10's.

I know three 10's minus one 10 is equal to two 10's.

Then 31 minus 10 must be equal to 21.

Great, now let's subtract our ones, but I'm going to use the Make 10 strategy here.

I know that 21 minus one is equal to 20, so I can partition the six into one and five.

So, 21 minus one is equal to 20.

So let's just take a look.

What we did, we started with 31, we minused 10, and then we minused one.

We still have to subtract five.

We can't forget about that second part of the partition, but oh no, I don't have any ones left.

All I have are two 10's.

So I need to regroup.

I need to regroup one 10 for 10 10's.

So 10 ones.

One, two, three, four, five, six, seven, eight, nine, 10.

There we go, I have 10 ones.

So I still have 20.

Now I can subtract five.

One, two, three, four, five.

So 20 minus five leaves me with 10, 11, 12, 13, 14, 15.

So I have 15 left.

That means 31 minus six is equal to 31 minus 10, minus one, minus five, which is equal to 15.

Both sides of this equation are correct because 16 was partitioned into 10, one and five.

10 plus one is 11, 11 plus five is 16.

So we just partitioned 16 and still subtracted.

Let's try one more, but this time you get to do it with me.

If you have your pasta shapes, get them ready because you'll need them.

We're going to use spaghetti in place of 10's, and we're going to use our smaller penne or macaroni in place of ones.

So these are my 10's.

These are my ones.

I need three 10's.

10, 20, 30.

So if you have pasta, you can pause the video now.

Draw your whole part model and do what I'm doing.

Great, So we have 10, 20, 30, three 10's, and we need one one.

Now we need to subtract 17.

So I will partition 17 into one 10 and seven ones.

I know that three 10's minus one 10 is equal to two 10's.

So 31 minus 10 must be equal to 21.

Now I need to subtract my ones.

I know that 21 minus one is equal to 20, so I can partition seven into one and six.

So 21 minus one is equal to 20 and I still need to subtract six, but oh no, I don't have any ones left.

So let's regroup.

We'll regroup our one 10 for 10 ones.

One, two, three, four, five, six, seven, eight, nine and 10.

Now we can subtract our six.

So I know 10 minus six is equal to four.

So that means 20 minus six is equal to 14.

10, 11, 12, 13, 14.

So we subtracted six, which is equal to 14.

That's our missing part.

Again, both of these equations are equal because 17 was partitioned into 10, one and six.

So as usual let's do one together and then you'll be off on your own.

So I have the equation, 41 minus 14.

We are going to use the whole part model and the Make 10 strategy to solve the following equations.

So again, if you have pasta at home, now would be a good time to use it.

So I have the equation, 41 minus 14, the whole is 41, and we need to subtract the part of 14 to find the other part.

I can partition 14 into one 10 and four ones.

So 41 minus 10 is equal to 31.

Now I can use the Make 10 strategy.

31 minus one is equal to 30.

So I'll partition four into one and three.

31 minus one is equal to 30 and 30 minus three is equal to 27.

Pause the video.

Good luck.

Welcome back everyone.

Let's go over the answers together.

54 minus 25 was equal to 29.

41 minus 15 is equal to 26.

53 minus 25 is equal to 28.

41 minus 16 is equal to 25.

52 minus 25 is equal to 27.

41 minus 17 is equal to 24 and 51 minus 25 is equal to 26.

Good job everyone.

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

As always, don't forget to complete your final quiz.

And I really hope to see you next time.

That was really good work.

Bye everyone.