Around 1990 I had to do some interviewing and I used this kind of intentionally underspecified simple problem to weed people out. (It seemed radical at the time.) Did they realize the problem was underspecified? Did they try to elicit the underlying “why”? Did they collaborate with me to complete the spec? Were they able to restate the spec coherently? Could they articulate a workable implementation in some technology domain? Basic stuff. Interesting thing is that there were people who really didn't do very well, who we hired anyway, who went on to be reasonably productive and reliable developers. In retrospect I concluded the cognitive and social basis of their ability was incomprehensibly different from mine. But I'd still do those questions today.