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When we are asked to add or subtract numbers such as 125 + 75 or 125 - 25, we can often work out the answer mentally.
However, when we are given questions with bigger numbers, we usually need to use a written method.
In this activity, we will use the columnar method to add and subtract numbers.
Let's have a look at how we can use the method to add 362 + 623
First, we write the numbers in columns under the correct unit (hundreds, tens, ones).
It can help to put the biggest number on the top (when adding)
Remember to include a + sign so that you know that you are adding.
H T O
623
362
Starting at the right-hand column (the ones), we add each column and write the answers underneath.
H T O
623
362
985
362 + 623 = 985
Here is another example. This time we will need to carry over or exchange.
Remember to include the + sign so you know what you're trying to do!
H T O
4 2 5
5 5 7
9 812
First, the ones: 5 + 7 = 12
We can partition this into 10 and 2
The ten moves into the tens column and waits on the doorstep (see the small 1 which represents 1 ten).
The 2 stays in the ones column.
425 + 557 = 982
Now let's look at subtraction.
564 - 352
Remember to write a - sign so that you don't forget that this is subtraction!
H T O
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!
564 - 352 = 212
Now, let's have a go at subtracting with exchanging, also known as borrowing.
Again, remember to put the - sign so you don't forget that you're subtracting!
H T O
45 14 7
1 5 3
3 9 4
First, the ones: 7 - 3 = 4
Next, the tens: 4 tens - 5 tens we can't do, so we exchange (or borrow) one hundred for 10 lots of ten. This means that we now have only 4 hundreds in the hundreds column and 14 tens in the tens column.
14 tens - 5 tens = 9 tens (or 140 - 50 = 90). We write this in the tens column as 9.
Finally, the hundreds: we now have 4 hundreds - 1 hundred = 3 hundreds ( or 400 - 100 = 300). We write this as 3 in the hundreds column.
Our answer is 393
Phew, that was a lot to do!
Now, it's your turn to have a go using this method to add and subtract numbers. Good luck!
