Lesson video

Hello everyone.

I'm Mrs. Cayley, and I'm going to help you with today's lesson.

So today we're going to explain what happens when we add or subtract 10 to a multiple of 10.

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

So here's the outcome of today's lesson.

I can explain what happens when you add and subtract 10 to a multiple of 10.

Here are the key words for today's lesson.

Can you repeat them after me? My turn, add.

Your turn.

My turn, plus.

Your turn.

My turn, subtract.

Your turn.

My turn, minus.

Your turn.

Well done.

Look out for these words in today's lesson.

Here's today's lesson outline.

Today we'll explain what happens when you add and subtract 10 to a multiple of 10.

We'll start off with add and subtract 10, and then we'll move on to solving puzzles.

So let's start with the learning.

Here are some children that are going to help us today.

We've got Sofia and Jun.

Sofia and Jun have got some cubes.

This is one 10.

Can you see there are 10 cubes? This is one group of 10 or one 10.

Watch what happens.

How can you describe this? Can you see what happened? Sofia said, "We added another group of 10.

We now have one more 10." So how many 10s have we got now? It's two 10s, isn't it? Jun said, "We had one 10 and now we have two 10s." That's one more group of 10.

Sofia said, "We can say 10 plus 10 and write 10 add 10." You could have a go at writing that down.

So I wonder what 10 plus 10 is equal to.

Jun said, "We had 10 and now we have 20." We can see that 10 plus 10 is equal to 20.

We can see this on a place value chart.

Can you see we've got the 10s and the ones? And here we've got one 10 and no 1s.

We're going to add another 10.

10 plus 10 is equal to 20.

We started with 10 and we added one 10.

Now we have two 10s.

There it is on the place value chart.

Can you see what's the same and what's different? So first of all, we had one in the 10s column, and now we've got a two in the 10s column, so the 10s digit has changed from one to two.

Sofia said, "I know that two is one more than one." Jun said, "So 20 is 10 more than 10." The 10s digit increases by one when we add 10.

Can you see that the 10s digit changed from one to two? The ones digit has stayed the same.

It's still zero.

What do you notice here? Can you see two equations? We've got one plus one is equal to two, and 10 plus 10 is equal to 20, and we've got a stem sentence to help us.

If I know mm then I know mm.

Can we use one equation to help us work out another equation? Sofia said, "If one one plus one one is equal to two ones." So that's the first equation.

Jun said, "Then one 10 plus one 10 is equal to two 10s." That's the second equation.

Sofia said, "The sum is one more group of one in the first equation." And Jun said, "The sum is one more group of 10 in the second equation." So here's our stem sentence.

If I know one plus one is equal to two, then I know one 10 plus one 10 is equal to two 10s.

This is four 10s.

Can you see four groups of 10? Watch what happens.

How can you describe this? Can you see what happened? Have we got more groups of 10 or fewer groups of 10? Sofia said, "We removed a group of 10.

We now have one fewer 10." Jun said, "We had four 10s and now we have three 10s." So four 10s has now become three 10s.

Sofia said, "We can say 40 minus 10 and write 40 subtract 10." Jun said, "We had 40 and now we have 30." Jun said, "We can see 40 minus 10 is equal to 30." We can see this on a place value chart.

Can you see the four 10s here? We've got four 10s and no 1s.

Here's the equation.

40 minus 10 is equal to 30.

So we're going to subtract one group of 10.

We'll find out 10 less.

Sofia said, "There is one fewer 10." Jun said, "Now we have three 10s." Can you see it on the place value chart? What's the same and what's different with those two place value charts? So I can see we had four 10s to start with, and now we've got three 10s.

Sofia said, "I know that three is one less than four." "So 30 is 10 less than 40." The 10s digit decreases by one when we subtract 10.

So the 10s digit has gone down by one, hasn't it? What do you notice here? Can you see the two equations? Four minus one is equal to three, and 40 minus 10 is equal to 30.

We've got a stem sentence to help us.

If I know mm then I know mm.

I wonder what it's going to be.

Sofia said, "If four ones minus one one is equal to three ones," so that's the first equation.

"Then four 10s minus one 10 is equal to three 10s." That's the second equation.

The difference is one fewer group of one in the first equation, and the difference is one fewer group of 10 in the second equation.

Let's say the stem sentence together.

If I know four minus one is equal to three, then I know four 10s minus one 10 is equal to three 10s.

So 14 minus 10 is equal to 30.

And we can use the first equation to help us work out the second equation.

Let's check your understanding.

Use the fact to complete the equation and the stem sentence.

So the first equation says four minus one is equal to three, and the second equation is 40 minus 10 is equal to? I wonder if you can work it out and have a go at saying the stem sentence.

Pause the video while you think about this one.

What do you think about this one? So four minus one is equal to three.

Let's say the stem sentence together.

If I know four minus one is equal to three, then I know four 10s minus one 10 is equal to three 10s, and that's 30, isn't it? So 40 minus 10 is equal to 30.

Is that what you thought? Here's a different equation.

70 plus 10 is equal to? How can Sofia and Jun find the sum? Sofia said, "We need to find 10 more than 70." Jun said, "We can use our stem sentence to help us." The stem sentence says, "If I know mm then I know mm." What known fact can Jun use? Can you think of a known fact that can help with this one? Sofia said, "We can use 7 + 1 = 8." Let's say the stem sentence together.

If I know seven ones plus one one is equal to eight ones, then I know? What do you think it's going to be? Can you say the rest of the stem sentence? That's right, seven 10s plus one 10 is equal to eight 10s, so 70 plus 10 is equal to 80.

Sofia thinks she can be more efficient.

This means she can do it in an easier way.

Sofia said, "I know that plus 10 means I need to find 10 more.

When I see plus 10, I can think 10 more.

70 is seven 10s and has a seven in the 10s place." So imagine a place value chart with seven in the 10s column.

There it is.

"I need to increase the 10s digit by one to show one more 10." So instead of seven 10s, we'll have eight 10s.

"If one more than seven is eight, then one more 10 than 70 is 80." So 70 plus 10 is equal to 80.

Let's check your understanding.

Can you complete the place value chart to show 20 plus 10? Explain how the chart has changed.

So see if you can pause the video and think about what might happen on the place value chart.

What did you think about this one? So, we had two 10s to begin with, and we're going to add another 10.

So now we're going to have three 10s.

That's 30, isn't it? Is that what you thought? Here's a different equation, 50 minus 10 is equal to.

When we subtract a number from another number, we're finding the difference.

How can Sofia and Jun find the difference? Sofia said, "We need to find 10 less than 50." Jun said, "We could use our stem sentence to help us." If I know mm then I know mm.

What known fact can Jun use? Can you think of a known fact that might help him work out this one? Sofia said, "We can use five minus one is equal to four." Can you say the stem sentence? If I know five minus one is equal to four, then I know five 10s minus one 10 is equal to four 10s, so 50 minus 10 is equal to 40.

Jun uses Sofia's place value chart strategy.

So 50 minus 10 is equal to.

Jun said, "I know that minus 10 means I need to find 10 less.

When I see minus 10, I can think 10 less.

50 is five 10s and has a five in the 10s place." So here's the place value chart with five 10s.

That's equal to 50.

What's going to happen when we subtract 10? Jun said, "I need to decrease the 10s digit by one to show one fewer 10.

If one less than five is four, then one fewer 10 than 50 is 40." So the place value chart will become 40 instead of 50.

So 50 minus 10 is equal to 40.

Let's check your understanding.

Can you spot the mistakes here? Jun and Sofia have worked out some equations, but they've made some mistakes.

Can you pause the video and find their mistakes, and think about what they should be instead? Did you find any mistakes in their calculations? So we had 60 plus 10 is equal to 70.

That's correct.

70 plus 10 is equal to 80.

That's correct.

80 plus 10, is that equal to 70? No, 80 plus 10 is equal to 90, not 70.

Then we had 50 minus 10 is equal to 40.

That's correct.

40 minus 10 is equal to 30.

That's correct.

And 30 minus 10, is that equal to 40? No, 30 minus 10 is equal to 20, not 40.

I think they added 10 instead of subtracted, didn't they? Here's a task for you to have a go at.

Can you complete the equations and say the stem sentence each time? So here we've got 80 plus 10 and 80 minus 10.

Then we've got 60 plus 10 and 60 minus 10.

So see if you can say the stem sentence for each one.

If I know mm then I know mm.

So think about a known fact that can help you with these.

And what do you notice about your answers? Here's the second part of your task.

Here we've got some multiple of 10 number cards and some instruction cards.

Turn over a number card and an instruction card.

Complete the instruction.

If you are correct, you get one point.

First to five points wins.

Perhaps you could write the equation for each one.

You could try this on your own or with a friend.

So pause the video while you have a go at your tasks.

How did you get on with your tasks? Did you complete the equations? So first of all, we had 80 plus 10, that's equal to 90.

80 subtract 10 is equal to 70, 60 plus 10 is equal to 70, and 60 minus 10 is equal to 50.

Did you have a go at the stem sentence? So you may have said for the first one, if I know eight plus one is equal to nine, then I know that eight 10s plus one 10 is equal to nine 10s.

So for the second one, you might have said, if I know eight minus one is equal to seven, then I know that eight 10s minus one 10 is equal to 70.

And for the third one, you might have said, if I know six plus one is equal to seven, then I know six 10s plus one 10 is equal to seven 10s.

And for the last one you might have said, if I know six minus one is equal to five, then I know six 10s minus one 10 is equal to five 10s.

What did you notice about the answers? Jun said, "The 10s digit of the answer always increased by one when we added 10, and the 10s digit of the answer always decreased by one when we subtracted 10." How did you get on with the second part of your task? You might have done this.

I picked the number 40 and plus 10.

40 plus 10 is equal to? Well, I know that four plus one is equal to five, so 40 plus 10 is equal to 50.

Or you might have tried this, 10 less than 90 is? It's 80.

I know that 10 less than 90 is 80, because one less than nine is eight.

So one 10 less than nine 10s is eight 10s.

Let's move on to the second part of the lesson.

We'll be solving puzzles.

Jun and Sofia find a machine in the school maths cupboard.

It's called The Tenminator.

I wonder what it does.

Do you think it makes a noise? They find some number cards in its box, so they put one in.

Here, they've put the number 20 into the machine.

It makes a noise.

Squeak! I wonder what number's going to come out.

The machine whirs and spits out a number.

I wonder what number's going to come out.

The number 30 has come out.

What has the machine done? Jun said, "We put 20 in and 30 came out." Sofia said, "30 is 10 more than 20, so maybe the machine adds 10." Is that what you thought? How could Sofia check? Jun said, "Let's try another number.

If I put 40 in," what number do you think might come out? The number 50 came out.

Sofia said, "Then 50 comes out.

I am definitely right." So this machine The Tenminator adds 10 to a number.

20 plus 10 is equal to 30, and 40 plus 10 is equal to 50.

I wonder what other numbers could go into the machine and what would come out.

Let's check your understanding.

Say what happened with the machine and write the equation.

So get a piece of paper and a pencil ready to write down an equation.

So the number 70 is going to go into The Tenminator machine.

The machine whirs and spits out a number.

Crunch! The number 80 has come out.

Can you write down an equation to show what's happened? So think about what has the machine done.

Pause the video while you think about this one.

What did you think the machine had done? The machine added 10 to 70, so 70 add 10 is equal to 80.

Here we've got The Tenminator machine again.

Hiss.

The machine whirs and spits out a number.

The number 60 has come out of the machine.

What has the machine done? I wonder what number went into the machine.

Jun said, "Wait, did you put a number in?" Sofia said, "I did.

Can you work out what I put in?" I wonder what number she put into the machine.

Jun said, "Something plus 10 is equal to 60.

I know the machine adds 10, but we don't know what went in." Jun said, "I know that 60 minus 10 is equal to 50 because six minus one is equal to five." Jun said, "If 50 is 10 less than 60, then 60 must be 10 more than 50.

50 plus 10 is equal to 60.

Sofia, you put 50 in." "I did.

You used the inverse to find the answer." So when we're using the inverse, we're going in the opposite direction to find out the answer, and subtraction is the inverse of addition.

Five 10s plus one 10 is equal to six 10s.

Let's check your understanding.

Say what happened with the machine and write the equation.

The machine whirs and spits out a number.

The number 20 has come out of the machine.

What has the machine done? Pause the video while you think about this one, and write the equation to show what's happened.

What happened this time? The machine added 10 to 10 to equal 20.

So 10 plus 10 is equal to 20.

So the number 10 must have gone into the machine.

Here's the task for you to have a go at.

We've got The Tenminator machine again.

Can you fill in the missing numbers? Each time you can see the number that has gone in or the number that has come out.

The machine is adding 10 each time.

So can you try to fill in the missing numbers? Here's the second part of your task.

Jun pressed the button on the machine and the in and out parts moved.

Can you find the missing numbers now? Can you see this time we are putting a number in at the bottom and it's going back through the machine? And this time we're going to subtract 10.

So pause the video while you have a go at filling in the missing numbers.

How did you get on with the task? Did you fill in the missing numbers? So first of all, The Tenminator was adding 10 to a number.

So if 30 goes in, then 40 will come out.

And if 50 goes in, then 60 will come out.

And if 70 goes in, then 80 will come out.

Then you had to work out what went in.

We saw what came out and we had to work out what went in.

So if 50 came out, then 40 went in, and if 70 came out, then 60 went in, and 90 came out, then 80 went in.

Is that what you thought? How did you get on with the second part of your task? Did you work out the numbers that came out of the machine? This time we were subtracting 10.

So if 30 goes in, then 20 will come out, and if 60 goes in, then 50 will come out, and if 10 goes in, then zero will come out, and if 100 goes in, then 90 will come out.

Is that what you thought? We've come to the end of our lesson.

Today, we were explaining what happens when you add and subtract 10 to a multiple of 10.

This is what we found out.

10 can be added to or subtracted from a multiple of 10.

When you add 10 to a multiple of 10, the 10s number increases by one.

When you subtract 10 from a multiple of 10, the 10s number decreases by one.

Known facts can help you to add and subtract 10.

For example, if you know that three plus one is equal to four, then you know three 10s plus one 10 is equal to four 10s, which is equal to 40, and you can use this with different known facts.

Well done everyone.

See you next time.