I decided to play with the number 9

9+9 = 18

referring to principle in previous post Strange Math

1+8 = 9

now if we have 9x9 we get 81

8+1 = 9

again

9x1 = 9

9x2=18 and 1+8 = 9

9x3=27 and 2+7 = 9

9X4=36 and 3+6 = 9

9x5=45 and 4+5 = 9

also

9x456 = 4104 and 4+1+0+4 = 9

and so on and so forth

so it seems to me that if we multiply any number by 9 , when we add up the digits of the resultant we get nine

try it out and see whether this works