Hacker News Reader

I would phrase the puzzle differently.

They both need to say they know, otherwise we have no reason to believe Sandy has figured it out.

And they both need to be right, which we also have no reason to believe at the moment.

The puzzle doesn't state whether Sandy has figured it out because that information is not necessary to figure out the solution. We can infer that she has (under that implicit assumption that she is a perfect logician) by the fact that we know less than she does and still have enough information, though. As for that last part: we also don't truly know that Peter isn't lying just to mess with the puzzle, I guess. That is another implicit assumption.

Reply on news.ycombinator.com