Hey Vivian! It is indeed quite confusing, but it really all comes down to the wording of the question. If the question asked "what is the probability that BOTH are boys if one is a boy" then the answer would be 1/3. This is because by saying "both" you are removing the independence of gender between the two kids.

However in this case, the question is simply asking what the probability is that the second kid is a boy. Because it is independent of the gender of the first child, there's only two options, a girl or a boy. Therefore the odds is 50%.

I hope this was helpful!