[tropicalia]

You're right and I stand corrected - it's not that hard of a problem. I found this out when (sometime later) I actually sat down to solve it, without considering any of the hints posted elsewhere in this thread.

At least there seems to be some general agreement (within the thread) that, while not super-hard -- it's still basically a "eureka problem" -- that is, even among what we may consider to be adequately good engineers -- some will get the visual solution more or less right away; but a fair percent (25 or more) won't.

And that's given ideal circumstances -- working alone and in unhurried circumstances. Throw in the (considerable) distraction of having to think aloud in front of other people (who themselves may not exactly be providing the most relaxing vibe, to say the lease), and all the other stress that comes with interviewing -- and I'll bet that 25 percent failure rate rises quite considerably, in turn.

And in many cases -- if you fail just one of these "eureka" problems -- and maybe fail to provide the expected canned response to a "behavioral" question or two -- than that's pretty much all it takes. To be considered, you know, "not a fit".

As to why I jumped the gun -- fatigue, probably, from being fed a few too many "cute" problems like these.