Question 30
For each positive integer $n,$ let $$
a_n=\frac{(n+9)!}{(n-1)!}.
$$ Let $k$ denote the smallest positive integer for which the rightmost nonzero digit of $a_k$ is odd. The rightmost nonzero digit of $a_k$ is
a_n=\frac{(n+9)!}{(n-1)!}.
$$ Let $k$ denote the smallest positive integer for which the rightmost nonzero digit of $a_k$ is odd. The rightmost nonzero digit of $a_k$ is