These type of questions are perfect candidate for any developer interview. I would say, this particular question is in easier side of the interview, probably for Entry level or Junior level position but definitely helpful to practice such questions.
As far as Kalikopoi's question, its a good problem to think about and try to solve at home but rarely any interviewer will ask in real interview. Most interviewer will have 1 hour or less time to test the candidate and in that time we have to test many aspect of candidate including logic problems. Problems that takes very long time are usually skipped.