What is $4{,}567{,}891 \bmod 9$?

Explanation

1. 01

   $10 \equiv 1 \pmod{9}$, so every power of 10 is $\equiv 1$, and a number is congruent to the sum of its digits.

2. 02

   $4 + 5 + 6 + 7 + 8 + 9 + 1 = 40$, and $40 \bmod 9 = 4$.