I think one of the things that trips people up with puzzles like this (including the one mentioned in https://news.ycombinator.com/item?id=9437483 ) is that knowledge of lack of information is itself information, a point that isn't used very often; if someone tells you they don't know something, you are likely to see it as simply a lack of knowledge, and not as a piece of information that could be combined with the information you have. It somewhat reminds me of that quote about "known knowns" and "known unknowns"...