If a is used to represent natural number, then odd number is? 0, odd number or even number?

If a is used to represent natural number, then odd number is? 0, odd number or even number?

If a is used to represent non-zero natural number, then even number can be expressed as (2a-2) and odd number can be expressed as (2a-1)
0 is even
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In natural numbers 1 ~ 20, odd numbers have (), even numbers have ()

In natural numbers 1 ~ 20, odd numbers are (1,3,5,7,9,11,13,15,17,19), even numbers are (2,4,6,8,10,12,14,16,18,20)

The sum of 100 natural numbers is 10000. In these numbers, the odd number is more than the even number, so how many even numbers can there be at most

100 / 2 = 50, odd number is more than 51, but sum is even number, so it can't be 51, at least 52. So even number can have 100-52 = 48 at most