Step by step
The question
Question: work out 45 + 17 + 55 + 23 in your head.
1 Find the easy pair
Look for two numbers that make a round number.
45 plus 55 is 100.
That is easy.
2 Change the order
We move 55 next to 45.
In adding, the order does not matter.
This is the commutative property.
3 Group the easy pair
We put brackets round the easy pair, so we do it first.
With only adding, we can group the numbers any way we like.
This is the associative property.
Careful!
This works for adding only.
With minus or divide, the order matters.
4 Work it out
45 plus 55 is 100.
100 plus 17 is 117.
117 plus 23 is 140.
5 Check
Let's check the long way, from left to right: 45 plus 17 is 62.
62 plus 55 is 117.
117 plus 23 is 140.
The same.
So the answer is right!
Let's look back.
First: find the easy pair.
Then: change the order.
Then: group the easy pair.
Then: work it out.
Last: check.