For whole numbers within 100, an even quantity can be partitioned into equal groups of two with none left over, while an odd quantity leaves exactly one unpaired; zero is even because it forms no incomplete pair. Paired objects, two-row arrays, and repeated addition connect odd and even groupings to multiplication and division by 2; generalized parity rules for larger numbers and symbolic algebra are not included.
To tell whether a whole number is odd or even, make groups of 2.
Example: Is 13 odd or even?
Start with 13 objects:
● ● ● ● ● ● ● ● ● ● ● ● ●
Pair the objects:
(● ●) (● ●) (● ●) (● ●) (● ●) (● ●) ●
There are 6 complete pairs and 1 object left over.
Since exactly one object is unpaired, 13 is odd.
We can also show this with division:
, so 13 makes pairs with 1 left over.
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