Lesson video

Hello, it's Miss Sew, and today, we're going to be working together to add decimals.

Before we start adding decimals for today's learning, I was thinking about the adding I do around my house, and I think that everyday I try and add up to five fruit and veg, my five a day.

I've got an apple, a banana, and an orange.

Three out of five of my five a day.

I've got the fruit, but I haven't eaten it yet.

Part way there.

Hope that you've been eating some fruit and veg lately.

Anyway, let's get back to learning our maths.

Make sure you're in a quiet space, and you're ready to learn.

I'm really excited that we're going to be learning all about decimals today at Oak Academy.

Welcome to today's lesson, all about adding decimals.

We'll be using column method to add our decimals today, so you will have to use your prior knowledge from other maths lessons to help you.

To start with, we'll be doing column addition with whole numbers, which you are familiar with.

After that, I'll be showing you how to apply this method to adding decimals.

Then, we'll be exploring some mistakes that I've made.

Silly Miss Sew, you'll have to help me, and then it'll be time for your independent task and quiz.

You will need pencil and paper to get started with your lesson today.

If you don't have those objects, pause the video, and go and get them now.

Make sure in a quiet place that is helpful for you for your learning, and you've turned off any notification on any apps you have running during this lesson.

To start with, we're going to warm up with some column addition.

Take a look at my three equations here, pause the video, and get started.

Right, are you ready for the answers? I'm going to show you now.

You have used column addition in previous maths lessons.

If you're finding this start of the lesson tricky, go back and look at some other Oak Academy lessons to help you.

You need to understand column addition before you can move forward with adding decimals.

You may have noticed in the last two questions, you've had to regroup.

You had to exchange 10 ones for one 10, or 10 10s for 100, and we regrouped by putting these numbers underneath in the correct columns.

Now, when we add decimals, we have to use a similar method.

Let's talk through what we did.

For these two equations, I'm going to have to regroup, and that's why I've chosen to use column addition.

I know that I have to regroup for doing six ones, add five ones, and four 10s, and seven 10s.

I could do this mentally, but it's a little bit tricky, so column addition is my preferred method.

To start with, I have six ones and five ones, which is equal to 11.

One one goes in the ones column, and the 10 goes in the 10s column.

Next, I have four 10s, add seven 10s, which is 11, plus the extra 10 put at the bottom, which is 12.

The two goes in the 10s column, and the one goes in the 100s column.

I've regrouped this equation.

Next, I have three 100s, add 200s, and the extra 100 at the bottom, which is 600s.

The answer is 621.

These numbers, 346 and 275 are quite large.

If I was thinking of a real life context, I might use numbers like 346 and 275 when I'm looking at a map, looking at going between two cities, and looking at the distance between them in kilometres.

Maybe for two places that are quite far away, I'd be adding up the distances, and trying to work out how far away it is for me.

Let's take a look at our decimal numbers.

To start with, I've made sure that my two numbers, 3.

46 and 2.

75 are in the correct place value columns.

This is really important when we add decimals.

I have 600ths and 500ths in the 100ths column.

I have four 10ths and seven 10ths in the 10ths columns.

My decimal points are all aligned.

They're in a straight line.

This is very important for adding decimals, and I've got three ones and two ones in the ones column.

Let's use the same method as before where I'll be regrouping and moving my numbers into different columns.

600ths plus 500ths is 1100ths.

100th goes in the 100th column, and one goes in the 10ths column.

I have four, add seven.

Four 10ths, add seven 10ths is 11 10ths, plus the one 10th at the bottom is 12 10ths.

The two goes in the 10ths column, and the extra one goes in the ones column.

I need to make sure I put my decimal point in the answer.

Without the decimal point, it will be the wrong answer.

I must do this and look really carefully.

Next, I have three ones, add two ones, which is five.

Add the extra one, which is six.

My answer is, can you say it out loud with me? 6.

21.

Thank you.

My answer is 6.

21.

6.

21, if I was thinking about it as a measurement, if I was thinking about 6.

21 centimetres, it's quite a small number.

It's the sort of length I could hold in my hand, and the sort of length I would use a ruler to measure.

6.

21 is 6.

21 centimetres isn't a very large number, and that's why I'm using decimals, so I can be more accurate and precise with my measurements.

Compared with my larger numbers, my 100s, 10s and ones, which were measuring distances between cities, it's a much larger distance.

Let's look at that answer more carefully, and think about what was really happening by using our place value chart to help us.

When I did, 600ths, add 500ths, I then had 1100ths.

I need to regroup the 1100ths, and put one 10th in the 10ths column.

I would take 10 100ths, and regroup them for one 10th.

I can wave bye bye to all of these 100ths, and replace them with just this 10th, which I will put in the 10ths column.

I now have one 10th and 100th.

They have the same value as 1100ths.

Now, let's look at my 10th column.

I have four 10ths, add seven 10ths, which is 11 10ths, plus the extra one, which is 12 10ths.

I can do the same.

I can regroup 10 of these 10ths into one one, and place this into the ones column.

Bye bye to all those 10ths.

I've now exchanged them for one one, and I have two 10ths left in the 10ths column.

I now have got five ones from adding three ones, add two ones, which is equal to five ones.

If I add the extra one at the bottom of my place value chart from regrouping my 10ths, I now have six ones.

The answer is 6.

21.

Six ones, two 10ths, and 100th.

I'm going to need your help with some common mistakes that happen when we add decimals.

Let's look at this equation here.

6.

54, add 13.

From looking at this number, I'm going to start to estimate.

Six ones, add 13 ones.

Mmm, six and 13 is equal to 19, so I'm looking at a number around 19.

I know that 6.

54 has six ones, and 13 has three ones, and one 10.

Now, looking at how these have been aligned, it doesn't look quite right.

If I did this equation now, four and three would be seven.

Five and one would be six, and then I would have six here.

The answer would be six, point, hmm.

I estimated that the number should be around 19, and that doesn't make sense, so I need to check my place value.

I have got 10s, ones, 10ths and 100ths.

In 6.

54, I have six ones, five 10ths, and four 100ths.

I need to make sure the decimal point goes into the right place, and is lined up with the place value chart.

Next, I have 13.

I know I have one 10, and three ones.

This looks much better, but I know that some of you will think, "Oh, there's a blank space there.

"What do I do?" We know that if there are no digits in the number, then the place value chart would put a place value inside.

There are no 10ths, and there are no 100ths, so I can just put zeros here as placeholders.

I must include my decimal point, and that's really important.

After lining the digits and the place value houses, I am ready to add up and solve this equation.

I need your help.

Some other mistakes have been made.

Which equation has been correctly lined up? The pink one or the purple one? Look really carefully at my question.

3.

16, add six.

Which one of these has been correctly aligned? Pink or purple? Point to your screen, and show me which one is correct.

It's the purple one.

There are six ones, so this six needs to go into the ones column.

If we look at this place value chart, there are three ones in 3.

16, and six ones.

I need to make sure my ones both go in my ones column.

Take a look at this question.

Which has been correctly aligned? The pink or the purple one? 2.

63 and 1.

2.

Point to your screen.

Show me which one you think is correct.

Look really carefully at the place value.

It's the purple one.

I have got 1.

2 with one one and two 10ths and 2.

63.

This person has left the 10ths column blank.

They could put a placeholder here, which would be zero, but that would change the number.

1.

2 and 1.

02 and numbers of different values.

Last one.

Pause if you need some time to think.

1.

79, add 12, which has been aligned correctly? Point to your screen.

It is the pink one.

12 has got one 10 and two ones, and 1.

71 has one one.

In this answer, I've put my 12 in the 10ths and the 100ths column, which doesn't make sense.

In 12, I know I have one 10 and two ones.

Let's explore by spotting another common mistake.

Here I've got 0.

8, add 0.

7.

It seems strange that the answer is less than the two numbers being added, so I know this is definitely a mistake.

If I add 0.

8 and 0.

7, I should have a greater number, not a smaller number.

One 10th is smaller than seven 10ths and eight 10ths.

There's no way this is the correct answer.

From my previous learning, I know I can use my known facts.

If I know that eight and seven is equal to 15, then I know that 0.

8 and 0.

7 is equal to 1.

5.

In my answer here, I haven't regrouped.

I know that 0.

8 and 0.

7 would be equal to 15 10ths.

I can regroup 10 of those 10ths for one one, and have five 10ths remaining.

The answer is 1.

5.

I'm going to need your help again to spot my mistakes, but I also want you to practise adding up these decimals.

Get your paper ready, and your writing tool, and make sure you are ready to have a go at these equations.

Pause the video, and have a go, and see if you can spot the mistake that I've made.

Time for the answer.

The correct answer is 4.

56.

This person has done their regrouping, but they forgot their decimal point.

The answer is not 456.

That's a greater number.

The answer is 4.

56.

Have a go at this question, pause the video, and write down your decimal addition on your paper.

Okay, time for the answers.

29.

04.

This person has not lined up their decimal points.

They made this mistake.

Remember, our decimal points have to be lined up straight.

I can tell this is a mistake, because I know that one, if I just look at the ones, one, and then the 10s and the ones here.

One, add 27 is about 28.

The answer should be 28, 29, something close to there.

Definitely not something above 30.

Have a go at this question.

Pause the video, and try your column addition.

Okay, you're ready for the answer? The correct answer is 4.

56.

This person has made a mistake.

They haven't regrouped.

They did two, add one is three, and they put it in the 10s column instead of adding it into the ones column.

I can also tell this is a mistake from my estimating.

Let's just look at the ones again.

Two, add one should be around three.

Two ones, add one one.

Three or four, definitely not 30.

There's not any 10s in this number, so to get from two, add one is equal to 30.

I know that's a mistake.

Make sure you check your answer to see if it's reasonable.

Now, it's time for your independent work.

Thank you so much for joining with the lesson today, and make sure you now make sure you have a quiet space, and have a go at these decimal equations.

If you found the last task tricky, rewind the video, and listen again.

Otherwise now, pause the video, or go to the worksheet, and have a go at this independent task.

Here are the answers to the independent task.

Thank you so much for joining in with your lesson today.

Share your work with Oak National.

If you'd like to, please ask your parent or carer to share your work on Instagram, Facebook or Twitter by tagging @OakNational or #LearnwithOak.

Well done for your fantastic learning today, and good luck for all your other lessons.

Make sure you go take my quiz to check how much you've learned.

Bye.