[tropicalia]

Even if you do not expect them to solve it.

The unstated implication in most cases is that you damn sure are supposed to solve it - or get "flushed".

As to "seeing how the candidate behaves in front of a difficult and concrete problem" - I'm sure this is what a lot people who ask questions like these think they're achieving by asking them. But again, you have to ask yourself if that's what the questions actually do permit you to assess.

My sense is that it's not - and that questions of this sort are mostly, well - cheap gimmicks, basically.