I do not dispute that the content of the question amounts to a simple application of conditional probability. However, you paraphrase the question as if it were a trivial manipulation of real numbers that could be conducted by a 4 year old able to multiply and divide. And perhaps it may seem that way to you as a seasoned statistician.
I think where my concern lies is in how a question being self-study was used as a reason to close a post whose content was deemed too trivial, rather than it being obviously a homework question. The OP took the time to type out in LaTeX the referenced context of the query, rather than it being a screenshot, or other tell-tale signs that they were seeking to turn people into homework machines. They've also made a reasonable attempt to engage with your prompts, even if there has been communication failure in the sense that they were unable to successfully reconstruct the frame of reference from which you trivially see the (non)-problem.
You've said yourself that you were searching for a reason to close on the basis of the question not being sufficiently interesting. If the question is not in any material violation of Q&A policies (i.e. blatant homework) shouldn't it be for the community to decide whether the content of a question is interesting, worthwhile, or even sufficiently trivial to not merit upvotes, not warrant responses, or even to merit a downvote?
And doesn't the fact that another user DoubleOughtNot felt that it was appropriate to take the time to supply a further prompt to the OP run counter to a unilateral determination of what is and is not sufficiently interesting?
I am not trying to make an issue here, I value the work of the moderation team on this Q&A. I just feel that this particular decision (out of hundreds and thousands of spot-on moderator decisions) on the basis of what I have heard so far might not have been the right one. Unless there is something I've missed?