Question 25
At a fish market there are 10 stalls, all selling the same 10 species of fish. All fish were caught in the North Sea or the Mediterranean Sea, and for each species, each stall has only one sea of origin. A number $k$ of customers each buy one fish from every stall, in such a way that they have one fish of each species. Furthermore, we know that every two customers have a species of fish with different origins. We consider all possible ways of stocking the stalls according to the rules above.
What is the largest possible value of $k$?
What is the largest possible value of $k$?