Question 19
Define $n_a!$ for positive $n$ and $a$ to be $$
\begin{aligned} n_a!={}&n(n-a)(n-2a)\\ &\cdot(n-3a)\cdots(n-ka), \end{aligned}
$$ where $k$ is the greatest integer for which $n\gt ka.$ Then the quotient $$
\frac{72_8!}{18_2!}
$$ is equal to
\begin{aligned} n_a!={}&n(n-a)(n-2a)\\ &\cdot(n-3a)\cdots(n-ka), \end{aligned}
$$ where $k$ is the greatest integer for which $n\gt ka.$ Then the quotient $$
\frac{72_8!}{18_2!}
$$ is equal to