BACK TO WORKSHEETS
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
9 911
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!
