I think you're missing the point of the question which seems to get at whether the interviewee is familiar with/understands bayesian probability. For an excellent explanation you should see: http://yudkowsky.net/rational/bayes.
I don't see why knowledge of Bayesian probability is necessary for this question. I don't have anything more than a passing knowledge of Bayesian probability, but I was able to produce the correct answer and a reasonable explanation using what I believe to be classical probability: https://news.ycombinator.com/item?id=7001288
