Can someone smarter than me figure this out?

“Me mind iz boggled.

http://digicc.com/fido/”

“I think we already had a thread on this exact subject, forget what thread it was though.”

“yup, i remember this one”

“Yeah, I've seen this one before. It works. Don't know how, but it does..... It ended up figuring the number I circled in my answer, which was #9.

Buck, wadda mean, you can't figure it out??? How it works, or don't know how to get the thing to run???”

“Ha! Silly me! It doesn't always work! I just tried another number and it came up with the same answer as the first! pppphhhhttt”

“"I have listed all 3 digit numbers algebraicly and subtracted their
possible juxtapositions and found that the result is always divisible
by 9. One of the 6 posibilities might be:
100 h+10t+u-(100 t+10h+u)which equals 90h-90t which is divisible by
9. All such cases are divisible by 9.
So if I choose 872 and subtract 728, I get 144(sum of digits 1+4+4 is
divisible by 9). Now if you circle one digit and tell ne the other 2,
(example: circle 4 and tell me 1,4, then I know the sum must be 9, (
or a multiple of 9), 1+4=5 then it is easy for me to tell you you
circled 4 cause the sum must be 9. Of course there are exceptions,
such as if you choose 555 the the difference is zero and the plot
fails. That case is excluded in the instructions. There are others
also but I leave them to you as an exercise." -by Tony DeGennaro

If it wasn't for Tony we might have to sight intelligent design.”

“I did it twice and it was correct both times. And they were different answers.”

“It's magic.”

“I think mjc's Tony is giving you the same answer I got from looking at the site:

Basically any three or four digit number subracted from a number with the exact same digits will yeild a number whose digits add up to 9 (or a multiple of nine... either 18 or 27 - notice, the digits in numbers that are multiples of 9 always add up to 9).

All the program needs to do is add up the digits you give and subtract them from 9 (or if it is 9 or more, add the digits from the new total and subtract them from "9").”

“It works with two digit numbers also, too since we are in a base ten system.

98 - 89 = 9
87 - 78 = 9
52 - 25 = 27 => 2 + 7 = 9
91 - 19 = 72 => 7 + 2 = 9”

“it works with 127 digit numbers too”

“This came up before. It uses a concept called, "Casting out nines".”