Whoops! I misread the question. The one I heard was between 20 and 50, making it practically impossible to guess much besides 37. And yes, I forgot 39. Chatati aviti pashati.
Basically, the way it works (at least my way) is that you automatically think 30 out of necessity, and then the human brain naturally thinks of a number above that. People believe that 35 is too obvious, so they pick the next one, which is 37.
That’s the way it was explained to me by a magician, so I’ll take his word for it.
I have no idea how it could work not a mod’s way. Too many variables.