Lesson video

Welcome to your math lesson, with me, Mrs. Harris.

Together we're going to be looking at strategies for subtraction.

Here's what we're going to do.

We're going to begin with mental strategies for subtraction, move on to efficient strategies.

Then, that'll be followed by column subtraction and an independent task, you to show me everything that you've learnt.

You're going to need a few things, a pencil, maybe a rubber, three dyes and something to write on, a piece of paper, or a book.

If you don't have those things, pause the video now, now find them and then, come back to me.

We're all set, let's get over with our learning then.

So, we're starting with mental strategies.

We know what five plus four is, it's nine, but how is that going to help us with our subtraction facts? Well we know, that subtraction is the inverse to addition, and we've got all the numbers there we need to make some subtraction facts.

Already I can tell you that nine minus five is equal to four or nine minus four is equal to five, but I can put that fact into other facts.

I can derive other facts from this, and I can use even greater numbers.

I know because I know that five plus four equals nine, I know that 90 minus 40 equals 50, do you see the link there? And then I know that 9 million minus 4 million is 5 million.

They're not the only set of facts I can derive from that addition question.

I also know that 90 minus 50 is equal to 40 or 900,000 minus 500,000 is equal to 400,000.

There are still more facts I can derive from simply knowing that five plus four equals nine.

I know that 99 minus 45 is 54, 990 minus 540 is 450.

And I also know that 999 minus 554 equals 455.

So what we may consider as really simple addition facts, when we use them mentally like this, we can derive lots and lots of facts, subtraction facts from them.

So I'd like you to make use of known facts to solve 12,755 minus, subtract 6,252.

Want to let you pause the video to give you a second to do it.

Okay, so, here's how I worked it out.

I know that 12 minus six is six.

So 12,000 subtract 6,000 equals 6,000.

I also know that 75 minus 25 is 50.

So, 750 minus 250 must be 500.

Therefore my answer is you see it? 6,503.

I didn't even write the bit down where I had to take two away from five.

We know that or don't we? Well done, if that's how you solved it.

Maybe you had another way, but this is it quite good mental way of doing some subtraction.

We're going to have a look at some efficient strategies now.

One of the efficient ways we can do subtraction is by rounding.

Take a look at these numbers, which one would you round and where would you round it to? Well, I know that 12,991 rounded to the nearest 1000 is 13,000.

It's only nine less than 13,000.

So, what I can do is I can use my number line.

I can pop my first number on my 32,678.

I can subtract 13,000 from it, but then I have to remember to put that nine at that nine back on meaning that the answer to this equation is 19,687.

Hmm, I think it's time you had a practise.

I'd like you to apply efficient strategy for the attraction.

Maybe it might be that you can do it using nine facts, all you have to do some rounding and use a number line.

You're going to play against a partner, you're going to with the number 10,000.

Roll three dice, make a 3-digit number with the dice you, with the numbers you rolled.

Then subtract that number from your 10,000.

You can have eight goes each, so eight rounds and the person after that time closest to zero is the winner.

Let me show you.

10,000 minus the three digit number is equal to whatever we read it is.

So, if you rolled a six, a four and a three, you might choose to put the six first why would you do that? I wonder.

That gives us 9,357.

So that becomes our new total for the next time we roll the dice, okay.

So, pause the video and have a go at this game.

Welcome back.

What strategies did the winner maybe you use to help you win? And what strategies did you use to support your subtraction? Some mental strategies, or maybe some rounding.

Let's move on now to column subtraction.

We're going to begin column subtraction by first considering how we partition a number into its powers of 10, just like on a place-value chart.

So we have this problem.

So it takes Uranus 30,697 days to orbit the sun.

Neptune's orbit is 19,931 days less than that of Uranus.

So, we know our minuend, the number we're going to subtract from.

We're going to begin our column subtraction by petitioning one number into its powers of 10.

Almost imagine it on a place-value chart.

We've got this problem.

It takes Uranus 30,687 days to orbit the sun.

Neptune's orbit is 19,931 days less than that of Uranus.

And we need to find out how many days is Neptune's orbit.

Well, we know our minuend, the number we're going to subtract from, that's 30,687.

We're not going to partition that one this time, we're going to partition the subtrahend the number we are taking away and we will find the difference and that will give us Neptune's orbit.

So, if we partition 19,931 into its powers of 10, this is what it looks like.

We've got one 10,000, look how many thousands we've got, nine of them.

Nine 100, three 10s and one, one.

Can you see how that relates to the number? We're going to use them now to subtract from our minuend.

So, we know that we need to subtract 19,931 from 30,687.

And we're going to do that by taking the 30,687, the minuend, first of all we might subtract 10,000, then 9,000, then 900, then 30, then one.

That's our number of 19,931 all broken up into its powers of 10.

Still a little confusing though.

I'm going to put it into a number line.

Can you see where I started? We need to remember that we started with the greatest number, the 30,687.

I decided to take away the 30 first, then the 10,000, then the 9,000, then the 900 and finally, the one.

And that gives me the answer, 10,756.

That is the difference between the two orbits.

So, I waiting to find the difference again.

On mercury, one day lasts for 1408 hours.

on Venus, one day lasts for 5,832 hours, and we need to know how much longer one day is on Venus.

We're going to use a number line to support our answers.

So, pause the video and have a go at it now.

Did you have a go, well done for trying.

So, here's what I did.

I started with my 4,800, my 4,000 and well, thought again Mrs. Harris.

I started with my 1,408 but I had to find out what the difference was.

So first of all, I added a two.

I thought we were doing subtraction.

Oh, we're trying to find the difference, aren't we? So then, once I added the two, I got to 1,410.

I then decided that the next landmark was 5,000.

So I added enough to get me to 5,000, then enough to get me to 5,800, then enough, 32, to get me to 5,832.

Still doesn't tell me my answer though, does it? It doesn't tell me the difference.

I think I'm going to need to add all of them up.

And when I do add them up, I find that the difference between 5,832 and 1,408 is 4,424 days.

We found the difference, was there maybe a more efficient way of doing it? But it would have been ever so slightly more efficient if we'd added up some of the addends first before putting them together.

In our minds I mean, look, we could have made 3,592 plus 8,332, it just would have made it a little bit more efficient.

Always try and look for what you can do in your mind quite quickly.

So, now we're going to look at column subtraction.

I've got 27,884 and that's the number I've made on my place-value chart.

I'm not going to make 8,033.

I would get confused if I did that and probably end up doing some addition.

So I just make the minuend.

Now, I've already made a start, I need to take seven from my ones column but I only had four of them.

So, I have one of my 10s and I broke it down into 10 ones and move them ones to the ones column.

And as you can see, I've crossed off seven of them.

That means, I don't have eight 10s in my 10s column anymore, I only have seven.

Now in my number 8,377, I have seven 10s.

I need to take away seven 10s now.

So cross them off.

Next, I move on to my 100s column I've got eight 100s, I need to take away three of them.

So I'll cross three of them off.

Let's look at the 1000s columns now.

The 1000 column now, I have 7,000 in there but I need to take away 8,000.

So I'm going to break down my 10, one of my 10 000s into 10, 1000s.

And now, I can take away eight of them, which leaves me with one, two, three, four, five six, seven, eight, 1000s in the column.

And I have just one 10,000 left.

Now what we need to do is find out how many counters I've got in each column and that will give you my answer.

One 10,000, eight 1000, five 100s, zero 10s and seven ones and here is the answer.

So, now it's time for you to do your independent learning.

I've given you a table and I would like you to fill in the blanks using the information you're given and using some of the strategies we've looked at in this lesson.

So, here are your questions.

Have you found all the answers? Let's have a look at them together.

So the first one, the total attendance was 24,902.

I wonder what strategy you used to work that out.

what did you find was most efficient for you? Next, we had to find out the empty seats and filled, and we found out that there were 20,873.

Did you use the same strategy? After that, we took away 6,005 from 42,785 to give us 36,790, same strategy, different strategy? Did you partition one of the numbers may be? And then St.

James's park, we found that the total attendance was 31,087.

Was that subtraction? Yes, it was, wasn't it? Don't be confused by having the two numbers there.

Originally, you need to think what the empty seats were.

Don't fall in the any traps.

And now, how did you work out the last one? The number of empty seats at Wembley.

Did you get the answer? 260, huh it was quite full that day.

Thank you for joining me for your math lesson.

If you'd like to share any of your strategies with me, you could ask your parent or carer to share your work on Twitter tagging @OakNational and I'll be looking out for it.

That just leaves you one last thing to do which is the quiz.

You'll be doing a few more subtraction questions with multiple choice answers.

Bye.