its simple
every time you do that formula you gonna come up with an answer that is divisible by nine
so, 0, 9, 18, 27, 36, 45, 54, 63, 72, 81
anything after 81 is impossible to get.
now, look at the list, all those numbers I listed, have the same symbol.
so no matter what number you think of, you will always get the same symbol
After you've done the test, symbols on the list gonna change to a new one.
nice trick, but a repost