Do you have a hobby with numbers to kill time?
I usually add up numbers of a license plate in a traffic jam. I find great pleasure if it is 3 or a multiple of 3. 7 is irritating and other numbers are boring except 5 which a cool number, 9 is neutral cause it does not change the final result and 1 is a winner, which basically leaves 2 4 and 8 to be boring.
As you can tell I have spent too many hours behind cars.
I really got good at it and could tell the final result within a second.
3475 = 7+12= 10 = 1
9+whatever = whatever
9+3 = 12 = 3
There is actually a game online to guess the number which is based on this property of adding 9.
This has to do with the base 10 counting that we do Ax10^1+Bx10^0 + 9 = (A+1)x10^1+(B-1) which works out to the same.. ok this is not a perfectly written down proof but I think you get what I am saying.. Thus 9 just lands up moving a number from one digit to the other 23 + 9 = 32. Thus you see 9 is neutral and can be ignored...
Damn I missed my bus writing this blog... will have to walk to work..