When we are asked to add or subtract numbers such as 15 + 5 or 15 - 5, we can often work out the answer mentally ( in our heads ).

However, when we are given questions with bigger numbers we usually need to use a written method.

In this activity, we will look at the **columnar method** to add and subtract numbers.

Let's have a look at how we can use the method to **add** **342 + 442**

First, we write the numbers in columns** under the correct unit **(hundreds, tens, ones).

**H T O**

**3 4 2**

**4 4 2**

We need to add an **addition symbol** (+) to show that we are adding.

Starting at the right-hand column (ones), we add each column and write the answers underneath.

**H T O**

3 4 2

__4 4 2__

7 8 4

**342 + 442 = 784**

Here is another example. This time we will need to **exchange **(also known as 'carrying').

Remember to include a + sign.

**H T O**

**6 6 6**** **

__3 2 5__

**9 9****1****1**

When we add 6 and 5 the answer is** 11. **

We can **partition** this into** 10 and 1.**

The ten moves into the tens column ( see the small 1 which represents 1 ten). We can think of this little 1 as being 'on the doorstep' of the tens column!

The one stays in the ones column.

We then move to the tens column and add 6 + 2 = 8 and then add on the 1 that was left on the 'doorstep': 8 + 1 = 9

Finally, the hundreds: 6 + 3 = 9

Here's our answer: **666 + 325 = 991**

Now let's look at **subtraction.**

**564 - 352 **

Set it out carefully in columns, but don't forget to include a subtraction symbol (-) so we know what we're trying to do!

**H T O**

**5 6 4**

__3 5 2__

**2 1 2**

**564 - 352 = 212**

Again, we start with the ones column, but this time we subtract the bottom number from the top (4 - 2 is not the same as 2 - 4).

It is important that we remember this!

Now, let's have a go at subtracting with **exchanging,** also known as** 'borrowing'.**

**H T O**

4514 5

2 5 2

2 9 3

**The ones**: first, we subtract 2 from 5 to get 3

**The tens:** next, we can't subtract 5 from 4, so we exchange (or borrow) one hundred for 10 lots of ten (500 becomes 400 and 40 becomes 140)

Now, we can subtract 5 (50) from 14 (140) to get 9 (90)

**The hundreds:** finally, we subtract 2 (200) from 4 (400) to get 2 (200)

Phew, that was a lot to do!

Now, why don't you have a go at answering some questions using the columnar method? Good luck!