AMC 12 2017b Problem 25

A set of $n$ people participate in an online video basketball tournament. Each person may be a member of any number of $5$-player teams, but no teams may have exactly the same $5$ members. The site statistics show a curious fact: The average, over all subsets of size $9$ of the set of $n$ participants, of the number of complete teams whose members are among those 9 people is equal to the reciprocal of the average, over all subsets of size $8$ of the set of $n$ participants, of the number of complete teams whose members are among those $8$ people. How many values $n$, $9 \leq n \leq 2017$, can be the number of participants?

\textbf{(A)}\ 477 \textbf{(B)}\ 482 \textbf{(C)}\ 487 \textbf{(D)}\ 557 \textbf{(E)}\ 562