To calculate or estimate an amount or number we use the rule of rounding off.
I would like to share my slides I use to teach my students this simple rule.
Definition of Rounding Off
Let us round off the number 24 as an example.
On a number line, which two round numbers is enclosing 24?
It is the number 20 and 30.
How far away is 24 to each of these round numbers?
24 is 4 away from 20 and 6 away from 30 therefore 24 rounded off is 20 as it is the closer number.
You can now note that any number that ends in 1, 2, 3 or 4 will round down.
Examples:
63 will round off to 60
22 will round off to 20
14 will round off to 10
It will then be true that any number ending in 5, 6, 7, 8 and 9 will round up.
Note: the number 5 is in the middle and as a rule it will always round up.
Examples:
75 will round off to 80
16 will round off to 20
39 will round off to 40
and that is the rule of rounding off!
This is my wall poster (or as I like to call it a "memory peg") I have in my classroom for the students to remember.
Remember this simple lymerick:
"One to four.... go down to the floor;
Five or more.... raise the score"