Lesson video

Hello, it's Ms. Sew, and today we're going to be learning about subtracting decimals.

My favourite way of subtracting is, I get a whole mug of tea and I subtract it.

In the morning.

How do you start your day? So, let's get on with our maths lesson where we are going to be using column subtraction to solve some decimal equations.

Today, we're going to be subtracting decimals.

You are going to build on what you've previously learned in other maths lessons using column subtraction, and today we're applying it to decimal equations.

To start with, we are going to do guys a warm up with some column subtraction with whole numbers that you are familiar with.

After that, we'll be applying this method to subtracting decimals, and then you will have a go, having a go on your own and spotting some mistakes that I've made.

At the end of the lesson, there'll be time for an independent task and a quiz to check how much you've learned.

For today's lesson, you will need a pencil and some paper, and if you don't have those things, pause the video and go and get them now.

Take a moment to check you have a calm, quiet space to do your learning and that if you have any apps running, notifications are turned off so that you can concentrate on today's lesson.

So, for our warm up, we're going to be doing some column subtraction, this is a method that you should be familiar with already.

Pause the video and have a go now.

Okay, I think we are ready for the answers.

So, here are the answers to our warmup.

You will notice that for our last two equations, we had to do some regrouping.

You had to exchange and regroup from our tens and put an extra 10 into our ones.

Six is greater than two and so you couldn't subtract it, you needed to regroup from our tens into our ones.

So, we are going to apply this column subtraction method to our decimals.

Let's think about what we're doing first.

So.

Our first equation is 346 subtract 275.

I'm going to estimate my answer so I can see if I've got anything wrong as I started doing it.

346 is close to 350, I'm going to round it up, and 275 is close to 300, I'm going to round it up too.

So, 350 subtract 300 is 50.

That's my prediction, I'm going to pop that there.

I'm estimating my answer so I can tell whether I'm getting close, or whether I've made any big mistakes.

So first of all, I'm going to do six subtract five which is equal to one.

And then I can't do seven subtract four, seven tens is greater than four tens.

I'm going to have to regroup and exchange from the hundreds.

I'll need to take one of our hundreds so there's now two hundreds left in this column and put it into my tens column, so now I have 14 tens.

14 tens subtract seven tens is equal to seven tens.

And then two hundreds subtract two hundreds is equal to no hundreds, I have got 71.

So my estimate of 50 is reasonably close.

50 is 21 less than 71, so this is a reasonable answer.

Let's have a look at what that could look like in real life.

Maybe if I was measuring some water or jug.

Maybe if I had 346 mils in here and I wanted to take away 275 mills for a science experiment I would have to be really accurate and I'd pour out and check to see if I had 71 mills left.

Let's have a look at our next equation where we are applying the same method to decimals.

Before I begin, I want to explain how I've put the numbers into the place value column because this is really important when we subtract decimals.

Look at how in my number 3.

46 and 2.

75, these numbers both have hundredths, our six hundredths and our five hundredths.

They both have tenths, seven tenths and four tenths.

And they both have ones, two ones and three ones.

The decimal point comes after the one.

If we look how I laid out my column subtraction, my six and five are both in my hundredths column, my four and seven are both of my tents column, my decimal points are lined up straight.

This is really important when we use our column method for subtraction of decimals.

My decimal points are lined up straight and my three and my two ones are also lined up straight.

So, before I start this equation I'm going to have a prediction allowing me to estimate the answer.

This will help me know if I've gone completely in the wrong direction and my answer has a mistake in it.

So, I'm going to round up 3.

46 to 3.

5, and I'm going to round up to 2.

75 to three.

3.

5 subtract three is my prediction, and that is equal to 0.

5.

I'm going to stick my prediction over here so I'm going to see if that is close when I get to my answer.

So to start with, I am going to do six hundredths take away five hundredths, that is equal to one.

Now I need to look at my tenths column.

I can't do seven tenths subtract four tenths because seven is greater than four, I need to regroup.

I am going to regroup one one and put it into my tenths column.

So I now have two ones, and instead of a four tenths, I now have 14 tenths.

14 tenths subtract seven tenths is equal to seven.

And now I get to put my decimal point in, really important these are all lined up, and now look at my one.

Two ones subtract two ones is equal to zero ones, and the answer is 0.

71.

My prediction was 0.

5, which is close to 0.

71, they are both numbers that are less than one.

If I'd have had an answer like 71 I would've known that would be completely incorrect because if I'm subtracting two decimals and I estimated, I knew my answer would be less than one.

If you forget your decimal point and your answer is in the tens, check your estimation because then you know you've made a mistake.

3.

46 subtract 2.

75 might be something that I'd do if I was measuring.

If I was measuring some string with a ruler and I wanted to cut a bit off.

I could subtract it by cutting away 2.

75 and seeing how much I'm left with.

It's a really small amount if I'm measuring in centimetres and I might need to be really precise if I'm doing it for a craft project and I'm making something.

So, let's have a closer look at what we're doing when we subtract those decimals.

To start with, let's look at how I have lined up my digits in my column subtraction.

There are six hundredths in 3.

46, and five hundredths in 2.

75.

There's a six and a five in my hundredths column.

There's four tenths in 3.

46, and seven tenths in 2.

75, and I've put these in my tenths column.

I have lined up my decimal points so that they are straight and my ones are also in the same column.

Let's start by looking at our hundredths over here, our blue counters.

Six hundredths subtract five hundredths is equal to one hundredth.

I can write one in my hundredths column in my column subtraction representation here.

Four tenths subtract seven tenths.

Hmm, I can't do that because there's not enough tenths, I will have to regroup.

I'm going to cross out one of my ones and then I will move it into my tenths, so now there are two ones left and there's a one in my tenths column.

I can regroup one tenth for 10 tenths and I will have 14 tenths now in my tenths column and now I can subtract seven tenths.

There are seven tenths left and I've put that into my column subtraction.

Now I have two ones subtract two ones and I have to put my decimal point in first which is equal to zero ones.

The answer is 0.

71.

Let's have a look if we can spot some common mistakes in column subtraction, I'm going to show you first.

I have got 1.

62 subtract 0.

7.

If I have one whole and six tenths and I am subtracting seven tenths from six tenths, I would expect the amounts to be less than one, but this answer is 1.

12, it's greater than one, that seems strange.

Now, I've done two subtract zero, there's nothing here, which is the same as a zero, which is two, that's correct so far.

But then six tenths subtract seven tenths is not one.

If I have six and I subtract seven.

I don't have another finger.

I can't do that, I need to regroup.

So what I should have done is, regroup from my one whole and do 16 subtract seven, my answer should be 0.

92.

I'm showing my regrouping into this representation now, the answer should be 0.

92.

So if this person has forgotten to regroup, let's make sure we don't make the same mistake when it's our turn.

So, I want you to have a go now.

Get your pencil and your paper and have a go doing this equation.

Can you spot the mistake that's been made and also have a go doing the column subtraction yourself? Pause the video now.

Okay, let's have a look to see if your answer was correct and if you spotted the mistake.

The answer should be 0.

99.

This person did not line up their decimal points correctly.

Our decimal points need to be in a straight line just like we discussed earlier in the lesson.

When we line up our digits into the correct place value columns, we can solve this equation correctly.

Have a look at this next question.

Have a go yourself on your paper and see if you can spot the mistake, pause the video now.

Okay, let's see if our answer was correct.

So, the correct answer is 6.

289.

Hmm, let's see what this person did wrong.

Four subtract five is not one.

If I have four and I want to subtract five, one, two, three, four, I don't have enough fingers, I need to regroup.

They should have what the answer is 6.

289 by doing this.

They need to regroup from their hundredths and put one hundredth into the thousandths column.

14 subtract five is equal to nine.

Then we would have seven hundredths and I would need to subtract nine hundredths.

I can't do this either so I need to regroup again.

I would have five tenths instead of six tenths and put an extra tenth in my hundredths column, so I would have 17 subtract nine.

Then I could do five subtract three from my tenths and then seven subtract one for my ones.

The correct answer is is 6.

289, this person forgot to regroup.

So, now the time for you're independent task, you have got four equations here that all require subtraction using the column method.

If you finish, can you have a go at my challenge and create a math story with these examples? Just like I showed you before, I could be subtracting water for a science experiment, or I could be measuring strength to cut as well.

In my example below, for my maths story, I've created a maths story for this equation.

Miss Sew had 6.

4 metres of ribbon, She cut off 2.

6 metres to make a bow, how many metres of ribbon did she have left? So, if you finished these equations, have it go a maths story? Pause the video now and go to the worksheet on the next page or look at the video.

Okay, time to show you the answers.

Here are the answers for the independent task.

Did you have a good maths story? I would love to hear them.

So, if you'd like to, please ask your parent or carer to share your work on Instagram, Facebook or Twitter tagging @OakNational and hashtag "Learn with Oak".

Share your work with Oak National.

Now it's time to complete your quiz for all our learning today.

Thank you so much for joining in, I hope you have a fantastic day of learning with all your other lessons, bye.