Teacher talks: Video playlist

[THEME MUSIC]

[SCHOOL BELL RING]

Hello, I’m Mr Smith, and together we’re going to practise subtracting one-digit numbers from one-, two- and three-digit numbers.

Finding the difference, taking away, counting up, counting back — these are all words that describe subtraction.

There are many ways we can subtract one number from another.

We can solve a calculation in our head, or work it out on paper.

Let’s start by looking at questions we can solve in our head.

Can you solve nine subtract three in your head? What is it?

Excellent!

Nine take away three is six.

But how did you solve this calculation?

One method is to count back from nine.

Nine,

eight,

seven,

six.

In this example we are subtracting or taking away three from nine.

We could also count up from three to nine and keep track of how many units it takes to get to nine.

Three,

four,

five,

six,

seven,

eight,

nine.

In this example we are finding the difference between three and nine.

Let’s look at another subtraction question.

What is 34 subtract eight?

Remember, you can count up or count down in your head.

Ready?

Using the taking away or counting back method, I know I need to take eight away from 34.

That’s a bit tricky. So, let’s break up eight into two lots of four.

34 subtract four equals 30.

And 30 subtract four is 26.

Did you get 26 too?

It’s really helpful to remember that we can break up or partition the second number to make the calculation easier.

We can also use a number line to help us keep track of our thinking too.

Can you solve this subtraction question?

What is 123 subtract five?

Have a go at solving it.

Here is part of a number line to help record your thoughts too.

Ready for the answer?

Excellent.

First, let’s use the subtraction method of counting up. And there’s a fair bit of counting up to do.

We are subtracting five from 123.

So, we start at five and jump 100 to 105.

Then, add five to get to 110.

And a jump of ten to 120.

And finally add three to 123.

Now, let’s add up those jumps to make 118.

So, 123 take away five is…

Phew!

That was a long way to subtract five.

Let’s try the other way and see if it’s quicker and check we get the same answer.

It’s often a good idea to use one method to solve a question and the other method to check.

Let’s have a go together.

This time I’m starting with 123 and I’m going to take away three so I reach 120.

But I need to take away five.

So, I need to take away another two.

So, 120 take two is 118.

There we have it.

We can use either taking away or finding the difference by counting up to solve subtraction questions.

Sometimes, however, one of these is much quicker than the other.