QUOTE(petrat @ Apr 10 2009, 11:32 AM)

Right, please will someone explain to me how the H3ll this works? I didn't write the numbers down anywhere and it KNEW! Weird!

Hope you can cope with using letters for numbers!
If the first digit is x and the second is y, the actual numerical value is 10x+y (think tens and units at school).
You then take away x+y (the sum of the two digits), which leaves a numerical value of 9x. So the answer is always a multiple of 9.