Lesson video

Hello, everyone.

And welcome to math with Ms. Dobrowolski.

If you haven't been here before welcome, but those of you who've seen me before, it's really nice to see you again.

So for today, we'll be looking at 2 digit numbers and ones with regrouping or subtracting 2 digit numbers and ones with regrouping.

So let's have a look at today's lesson agenda.

First we'll review the 'Make 10' strategy.

Then we'll be subtracting 2 digit numbers and ones with regrouping.

Followed by your independent task and your final quiz.

For this lesson, you'll need a pencil, a notebook, and possibly some pasta shapes.

Spaghetti and macaroni or penne would be best.

If you can't get pasta, don't worry.

It's totally fine.

You can still be successful in this lesson without them.

However, if you can get these items, pause the video now and go get them.

Great.

So let's have a look.

When can we 'Make 10?' Well, before I do that, I wonder, are there any known facts I can use to help me solve these problems, so I don't have to make a 10? Let's have a look at my three equations, 32 minus 5, oh 2 minus 5, that's not a known fact.

So can't use my known-facts to help me there.

What about 43 minus 7? Is three minus seven a known fact? No, because 7 has a greater value than 3.

What about 61 minus 5.

But for all three of these equations, I can see that when I subtract, I'll have to cross a 10.

32 minus 5 will have to be less than 30 and 43 minus 7 will have to be less than 40 and 61 minus 5 will also have to be less than 60.

So I can use my make 10 strategy for these.

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

Great.

So let's have a look at this first problem.

So I'm going to try and solve 32 minus 5.

Well, I know that I will cross a 10 because 32 minus 5 will definitely be less than 30.

Hmm, I know 32 minus 2 is equal to 30.

So that means I can partition or split the 5 into 2 and 3.

So I start at 32 and first I jumped back one, two that takes me to 30, just like I thought it would.

And now I need to jump, subtract three, one, two, three.

That takes me to 27.

So my answer must be 27.

Let's try the next one, 43 minus 7.

Again, I know my answer will be less than 40 because 7 is, has a greater value than 3.

So, hmm, 43.

Well, I know 43 minus 3 is equal to 40.

So I will partition the 7 into, 3 and 4.

Remember 3 plus 4 is equal to 7.

So that's why I can use this partition.

So I start at 43 and first I'll subtract three, one, two, three.

So that gets me all the way to 40.

Now I have to remember to subtract 4.

One, two, three, four.

That takes me to 36.

So my answer is 36 And let's try last but not least, 61 minus 5.

So we start at, let's see, 61.

Oh, do you know what number will get us to 60? Well, I know 61 minus 1 is equal to 60.

So I'll partition the 5 into 1 and 4.

Remember 1 plus 4 is still equal to 5.

So we start at 61 and we jumped back one or subtract one, I should say, and that gets us to 60 and we still have to subtract our four, one, two, three, four.

That gets us all the way to 56.

So 61 minus 5 is 56.

So another way we can represent these equations is by using a part whole model with partitioning.

So if 32 minus 5, and now I'm going to draw my whole part model, which you can really easily do at home.

First, you drop a big box and then you draw two smaller boxes to represent your parts.

So this would be your whole.

And these two smaller boxes are your parts.

So what is our whole? Well, I know 32 is our whole, because that's the number we're subtracting from.

So when we subtract, we only need to make the minuend or the start number by placing it here in the whole section.

So 32 has three 10s, 10, 20, 30, and two 1s, one, two.

Great.

When we use a whole part model, we subtract the 1s first to see if we need to regroup.

I can see that 32 has two 1s and I need to subtract five 1s, but I can't, rutroh 5 has a greater value than 2.

So I need to regroup my one 10 into 10 ones.

So let's get 10 ones, one, two, count with me.

Three, four, five, six, seven, eight, nine, ten.

And as you can see, 10 ones are equal to one 10.

So that's me regrouping.

I put 10 ones take away the one 10, and I still have 10 20, 30, 1, 2.

So I still have 32.

I've just regrouped.

Great.

So now it looks like I have 10, 11, 12 ones and I can subtract by.

So remember, I'll use my make 10 strategy and partition the 5 into 2 and 3.

And that's because 32 minus 2 is equal to 30.

Now I can't forget to also subtract 3, 30 minus 3, and now it looks like I have 10, 20, 21, 22, 23, 24, 25, 26, 27 left.

So that's my other part now.

So 32 minus 5 is equal to 27.

Great.

Let's try it with another equation.

So 43 minus 7.

What's our whole? Again, 43 is our whole, because that is the number we're subtracting from.

So when we subtract, we only need to make the minuend or the start number by placing it here in the whole section.

So 43 has how many tens? That's right, four.

10, 20, 30, 40.

And how many ones? That's right, three.

One, two, three.

Excellent.

Remember when we use a whole part model, we subtract the ones first to see if we need to regroup.

I can see that 43 has one, two, three 1s.

And I need to subtract seven 1s, but I can't, 7 has a greater value than 3.

So what do I need to do? I need to re-group.

So instead of having one 10, I'm going to replace it with 10 ones.

Count with me.

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

Now I have, I still have 10, 20, 30, 40 1, 2, 3.

I still have 43, I've just regrouped.

Now I have 10, 11, 12, 13 ones, and I need to subtract 7, but let's use the make 10 strategy.

So I know 43 minus 3 will make, is equal to 40.

So I'll partition 7 into, 3 and 4.

3 plus 4, still equal to 7.

So 43 minus 3 is equal to 40, 40 minus 4 is equal to 10, 20, 30, 31 32 33 34 35 36.

So after I subtract seven, I'm left with 36.

So 43 minus 7 is equal to 36.

Great.

Before we get started on this one, it might be a good idea to get your pasta shapes to hand.

We can use spaghetti to represent our tens, and we can use some macaroni or penne to represent our ones.

So pause the video now and get these to hand if you don't already have them.

Great.

So let's have a look.

What's our whole? Well, I know that 61 is our whole because it's the number we'll be subtracting from.

So remember, that's the number we make.

That's our minuend or start number.

So we place it here in the whole.

So let's get our spaghetti 10, 20, 30, 40, 50, 60.

So that's six 10s and one 1, 61.

Now, remember we start with the ones to see if we need to regroup.

Hmm.

Can I do 1 minus 5? No, I can't.

I need to regroup.

So I'm going to regroup my one 10 for ten 1s.

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

So I still have 10, 20, 30, 40, 50, 61.

I've just regrouped.

Okay.

Now I have 10, 11.

I have 11 ones and I can subtract 5.

But let's use our make 10 strategy.

So I know that 61 minus 1 is equal to 60.

So I can partition 5 into 1 and 4.

So I know 61 minus 1 is equal to 60.

And then 60 minus one, two, three, four leaves me with 10, 20, 30, 40, 50, 51 52 53, 54, 55, 56.

So after I subtract 5, I have 56 left.

So 61 minus 5 is equal to 56.

And it's already time for your independent task.

How exciting.

So as usual, I'll do an example before you go off on your own.

So I'm asking how much do these children have left? So my example is this boy, I'm going to name him Tom, Tom has 43 pence and he buys a rubber.

So the rubber costs 8p.

How much money does he have left? So I've drawn myself a whole part model.

And if you're at home with spaghetti and penne, you can use your spaghetti and penne to help you.

And I know that 43 is my whole because that's how much pence he has.

So I made 43 and I'm going to subtract eight pence because that's the price of the rubber.

So 43 minus 8 is equal to 35.

So Tom has 35 pence left.

And that will be my answer.

So for the rest of these one, two and three, you have to look at what they buy, figure out the calculation that you need to write.

And then tell me how much do they have left after they've bought their item? So your turn.

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

Good luck! Oh, I forgot to tell you, there's a challenge if you finish quickly or if you just want a little bit extra to do.

So the challenge is I started with 32 pence and now have 24 pence left.

What did I buy? Dump dump dump.

That's the mystery for you to solve? Well done everyone.

So let's go over our independent tasks first.

In number one, I see the child had 52 pence and bought his pencil for 7p.

So 52 minus 7 is equal to 45p.

Here we have, the girl had 74 pence and bought a comic book for nine pence 74 minus nine pence is equal to 65 pence.

And in number three, the boy had 21 pence and bought a pen for six pence.

So 21 minus 6 is equal to 15 pence.

And in our challenge, I started with 32 pence, that was my whole, so that's my minuend or my start number.

And now I have 24 pence left.

So 32 minus 24 is equal to 8 pence.

Ah, I must have bought a rubber because rubbers cost eight pence.

That was a tricky one.

So those of you who took on the challenge, well done for the effort.

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

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

And I really hope to see you for future lessons.

Well done everyone.

Bye.