k here goes, lemme know if its not clear...
the way it works is that you will always get a multiple of 9. if you look at the sign next to the number 9 (or any multiple of 9) youll see its the same, and thats the sign that pops up when yo u hit th button.
to see how it works, think it through this way: when you pick your number, lets say XY, we are going to be subtracting X+Y, correct? so lets subtract Y first... now we have X0, right? (for example 86, minus 6, is 80). so now we take away the first digit .. ie X0 - X. (example again, 80-8=72).
if you subtract the tens digit from any two digit number like that, you will get a multiple of 9:
10-1 = 9
20-2 = 18
30-3 = 27
etc...
if you want to really prove it (working from the point after we removed the 'Y'), think of it this way - its all the numbers in the form (10X - X), if you look atthe examples i gave up there...
10X-X = 9X, which is always (obvioulsy) a multiple of 9. which explains how i was able to say that first paragraph....
for the people who it didnt work for?.. not sure what to tell you
:dunno