Mind-reading Crystall Ball?

A lot of people have seen this online crystall ball that can read minds Crystal Ball. The first thing everybody invariable says is how does this thing work? Here is how it works (super intelligent people don't bash me, this is for people who wonder how this works)

We will follow the steps one by one, and then see how this works

Let us take a 2 digit number 10x+y
add the digits and subtract them from the original: 10x+y - (x+y)
=> 10x-x+y-y
=> 9x
x can be 1,2,3,4,5,6,7,8,9
=> it can be 9,18,27,36,45,54,63,72, or 81
all these numbers on the chart have the same symbol :)