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Students and problems with constraints, find number of students

Source: 2018 Latvia BW TST P8

June 5, 2022
combinatoricscombinatorics unsolved

Problem Statement

Let natural n2n \ge 2 be given. Let Laura be a student in a class of more than n+2n+2 students, all of which participated in an olympiad and solved some problems. Additionally, it is known that:
[*] for every pair of students there is exactly one problem that was solved by both students; [*] for every pair of problems there is exactly one student who solved both of them; [*] one specific problem was solved by Laura and exactly nn other students.
Determine the number of students in Laura's class.