>Which is about the right time - I heard it from my students, during early eighties. Isn't it strange how good jokes crossed borders effortlessly even before Internet?

Yes, interesting. I heard it from my sister about 20 years ago and it was a favorite "math" question of hers.

Mine was at some time about wolf who was running in the forest. But it's a bit hard to translate correctly into English, besides, I asked it couple of years ago here as well.