To quote my good friend Huminie who is more of a mathmetician (and speller too.

):
Here it the pattern:
The lowest 2 digit number you can have is 10. 10 through 19 all have the same answer to the formula which is 9.
10: 1 + 0 = 1 subtracted from 10 = 9
11: 1 + 1 = 2 subtracted from 11 = 9
12: 1 + 2 = 3 subtracted from 12 = 9
13: 1 + 3 = 4 subtracted from 13 = 9
etc all the way to 19.
The next group is 20 to 29.
20: 2 + 0 = 2 subtracted from 20 = 18
21: 2 + 1 = 3 subtracted from 21 = 18
etc all the way to 29
30 to 39:
30: 3 + 0 = 3 subtracted from 30 = 27
etc...
40 to 49:
40: 4 + 0 = 4 subtracted from 40 = 36
etc...
50 to 59 = 45
60 to 69 = 54
70 to 79 = 63
80 to 89 = 72
90 to 99 = 81
You will notice that everytime the page is refreshed the symbols for 9, 18, 27, 36, 45, 54, 63, 72 and 81 are ALWAYS the same.
It is all math, and it will always give you the right answer.