If a number's digets add to 9, the number is a multiple of 9.
09, 0+9 = 9
18, 1+8 = 9
2+7, 3+6, 4+5, etc.
Applies to any number of digets as far as im aware.
Therefore 111111111 is a multiple if 9. (It's 12345679 × 9 to be exact)
This us why I love math.
I rest my case.