In the context of the ticket sales for the fair, the variables \( x \) and \( y \) represent the number of adult tickets and children's tickets sold, respectively. Since you cannot sell a fraction of a ticket and negative ticket sales do not make sense, the most appropriate constraint for viable solutions is:
A viable solution will have only positive whole numbers.
This means both \( x \) and \( y \) must be non-negative integers (whole numbers equal to or greater than zero).