It is sad to say that if you can't answer 1 puzzle, they might come to the conclusion that you don't have any IQ :-) It was my 1st interview after very long time, I was rather surprised to be asked a puzzle, I was thinking too literally or too practically about the problem as my mind was not ready for these kind of Qs. It would be more appropriate if they had asked about any programming logic as that is what you use in your daily work.