Thread: [SOLVED] Puzzling puzzle

This puzzle cetainly has a mathematical solution, but darned if I can figure it out.........can someone with higher skills show me the solution???
http://www.learnenglish.org.uk/games...er-central.swf
Thank you, Marty

All multiples of 9 have the same symbol and you can only ever get a multiple of 9. This makes sense because when you increase the number you first thought of by 1, you do not alter the final number at all because the digit sum is also increased by 1. When you increase the number you think of by 10, the digit sum is increased by 1 and so the final number is increased by 9.