Divisibility by 9/Corollary

Corollary to Divisibility by 9
A number expressed in decimal notation is divisible by $3$ iff the sum of its digits is divisible by $3$.

That is:
 * $N = [a_0 a_1 a_2 \ldots a_n]_{10} = a_0 + a_1 10 + a_2 10^2 + \cdots + a_n 10^n$ is divisible by $3$

iff:
 * $a_0 + a_1 + \ldots + a_n$ is divisible by $3$.

Proof
From Divisibility by 9 we have that:
 * $N = [a_0 a_1 a_2 \ldots a_n]_{10} = a_0 + a_1 10 + a_2 10^2 + \cdots + a_n 10^n$ is divisible by $3^2$

iff:
 * $a_0 + a_1 + \ldots + a_n$ is divisible by $3^2$.

So: