Got it first time.
It had taken Dr. Duke well over a year himself, and he would always explain that the smarter you were, the longer it took to figure it out.
I have to disagree strongly with this, not just because I got it right so quickly but because there's something inherently perverse about claiming that a mental task can be harder for more intelligent people. I have no qualms with someone saying that it's possible to be very intelligent and still not be able to figure this puzzle out, or even that someone can figure it out without being very intelligent, but claiming an inverse correlation is ridiculous.