Yep, we've visited it before... All of the answers will generate certain variables (has to do with multiples of 9 if I recall), so when the symbols
change, it will always be on one of those numbers.... There are a few other such seemingly amazing ones as well.
This is a branch of stage magic known as Mentalism... If you've never seen a show of one, try to catch it...really blows your mind....
All the numbers that are divisable by 9 are the same symbol. Every number, when you add it's digets together, then subtract that from the original
number is divisable by nine.