I usually find that if I come out of a job interview thinking "aced that one", I don't get it. If I come out thinking "oh well, never mind", I get an offer. I like to think that its attitude that matters, and the ones that reject me see a sort of cocky attitude as things are going well. But maybe its just me and the weird way my life plays out.
I've had other interviewers ask me really abstract problems, such as how to calculate the number and types of elevators a particular-sized building needed. Honestly, I had no fucking clue. I'm a videogame programmer, not an architect. I reasoned it out as best I could, and obviously I didn't guess well enough.
These questions are not about a right answer, they are entirely about how you reason them out. Most companies that use them just want to see you thinking, and communicating your thought processes (ie. to see if you can think!).