You're not saying that, no, which is why I just wanted to explain how someone like me might approach the question presented by Dave's teacher. I would have figured it out, yeah, but from conditioning to think like other people (which is weird, and constraining) rather than my own intuitive deduction. It's like the mental equivalent of being forced to use your right hand instead of your left like schools used to do back when I was a kid. I was ambidextrous and they shut that shit down fast.
I would have figured it out, but only because I take no chances with new teachers. I examine every aspect of the test before I take it, just so I don't come upon any trick questions. The problem is that for someone like myself, and I'm certain many others, such a tactic would be fruitless because it doesn't teach you to think critically, only to understand that your teacher is going to try to trick you, and while that can be a valuable skill to learn, I don't think it's in most teachers lesson plans.