truBlu

Mods: This doesn't belong in this forum. Originally I was going to ask about flash and disassembly but since that isn't the case here, it would be better off in the lounge. Please move it there.

Someone sent this link to me.

http://digicc.com/fido/fido.swf

It's a puzzle that has you randomly select a 3 or 4 digit number and then subtract from it a different 3 or 4 digit number created using the same digits. You then enter your answer into the flash program leaving out a digit that you have selected. The program then figures out which number you have selected. For example, if I pick 8530 and then rearrange that number to form 5803, the difference is 2727. By entering into the program any combination of 2, 7 and 7, it figures out that you selected the number 2.

What I'd like to know is how this is done. I've been sitting here for awhile now trying to see what kind of pattern emerges when you subtract two numbers with the same digits but I just don't see the pattern. This isn't a homework question so knowing the answer isn't the end of the world. It would just be nice to satisfy my own curiosity.