Question 21
There are 234 visitors in a cinema auditorium. The visitors are sitting in $n$ rows, where $n \ge 4$, so that each visitor in the $i$-th row has exactly $j$ friends in the $j$-th row, for any $i, j \in \{1, 2, ..., n\}, i \neq j$. Find all the possible values of $n$. (Friendship is supposed to be a symmetric relation.)