Set of Odd Digits

[Feliz]

Use set-builder notation to denote the set of all odd digits.

Let O = set of odd digits

Textbook answer:

O = { x | x is an odd digit } = { 1, 3, 5, 7, 9 }.

Question 

Why is 9 on that list of odd numbers when 9 is divisible by 1, 3, and 9? 
 

[studiot]

See my answer to your other question then come back for more help if you still can't answer it yourself.

[Feliz]

4 hours ago, studiot said:

See my answer to your other question then come back for more help if you still can't answer it yourself.

I responded to your reply concerning even digits. What is your definition of an odd number? 

[pzkpfw]

19 minutes ago, Feliz said:

I responded to your reply concerning even digits. What is your definition of an odd number? 

What is yours?

... and what's your definition of a prime number?