Take note that each time all the multiples of 9 have the same symbol, and each time you access it, it changes to a different symbol. Otherwise you'd figure it out too quickly.
A two digit decimal expression really means 10x+y. If you add the digits together you have x+y. Now subtracting the x+y from the 10x+y yields 9x. So, the answer would always be a multiple of nine. The largest number would be 99. Nine +nine equals 18, subtracted from 99 would be 81. So, 81 is the highest multiple of nine that shares the same symbol as all the other multiples of nine.
You could think of it without the symbols as this exercise:
Think of a two digit number.
Add the two digits together.
Subtract your total from your original number.
Your answer is an even multiple of 9.
So, 9, 18, 27, 36, 45, 54, 63, 72, and 81 always share the same symbol. /ubbthreads/images/graemlins/grin.gif