With all due respect to the Anonymous, if you can't work your way through this problem on your own, I feel for the person who must prove their own value to you.
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In my view, interview questions are not to be doled out by the shovelful. You should decide what are the desirable characteristics of an applicant for the position, and then probe applicants according to those issues.

That seems a bit unfair.

It's perfectly reasonable for an interviewer to want to gauge an applicants abilities under pressure, ability to deal with unexpected problems, willingness to ask questions,and generic "problem-solving" skills.

In situations like that, the specific question is usually irrelevant, what does matter is that there are some number of "logic problems" that the interviewer has handy to see how the interviewee responds.

Personally, I like to have a few such questions handy. I make sure i have time to ask one, and then if there's extra time towards the end, ask more untill the time is filled.