A coworker approached you and goes “hi, I have two children and one is a boy…” and is promptly vaporized because he doesn’t fit the selection criteria of the problem statement, another approaches and goes “err hi, I have two children and one is a girl…” looks nervously at the vaporizer but is left standing. What is the chance their other child is a girl, who has not been vaporized?

If you phrase the question as “someone with two children tell you the gender of a random one, what is the chance the other is the same gender?” Chance is 50/50 because 50% will have BB or GG and the vaporizer isn’t active.