text: think about a two-digit number [ab]=10*a+b (thus the two digits are a and b), then [ab]-(a+b)=(10a+b)-(a+b)=9a.  Therefore your choice only depends on the first digit a.\n\nthey put the symbol at every 9 places, so for every a from 1 to 9, there is a symbol at 9a.\n\nYou will notice that the same symbol is at 9, 18, 27, 36, 45, 54, 63, 72, and 81 (the possibilities for [ab]-(a+b))