[wdmeldon]

I think merely asking this question would be qualification enough.

[throwawayinterv]

As I suggested in another comment, it might be in general bad advice to try to get people to ask clairfying questions about non-contextual interview problems. If you want to measure social adeptness / willingness to ask for clarification -- which are important! -- consider interview questions that have a "role playing" component or actively invite the interviewee to ask questions.

[cema]

In my experience, clarifying questions are important. In this case, for instance, we have two different problems (well, not too different, but still).