Well, I must have overestimated those numbers (or maybe those kind of riddles are less popular where the study was performed than where I come from). Still, it might be interesting to also ask participants “Have you encountered this question (or a very similar question) before?” with possible answers “Yes, and I remember the answer (or how to derive it”, “Yes, but I have forgotten the answer”, “Yes, but I was never told the answer” and “No”.