Hi,

I have the following problem and I don't know how to go about this... I would really appreciate if you could give me a hand. The problem says:

"Let $\displaystyle r \in \mathbb{N}$. Show that

$\displaystyle r \equiv \Sigma mod 9$,

where $\displaystyle \Sigma$ is the sum of digits of $\displaystyle r$ (in decimal representation of r)."