.... I'm buggered if I can remember the solution. http://www.office-humour.co.uk/pop.cfm?link=http://www.milaadesign.com/wizardy.html
Nines. When you subtract the singles and the tens from the total you're always going to be left with a multiple of nine. It doesn't matter if they change the symbols around every time as long as the ones for the multiples of nine are all the same.
think its to do with the number 9.. just say.. you pick 42, 4 + 2 = 6. 42-6 = 36 36 is a multiple of 9.. </p> works for them all something like that anyway </p>