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Saudi Arabia booklet 2024

Saudi Arabia 2024 counting and probability

Problem

With positive integer , let positive numbers are written on the board (not necessarily all different). It is known that any 4 pairwise different numbers from that list form an arithmetic progression. Prove that some number is written on the board at least times.
Solution
Let be the number of distinct values appear on the table, let them be Assuming by contradiction that no value appears at least times, then each value appears at most times. So we can count the amount of numbers on the board to get Consider the 5 arbitrary distinct values among them, denoted by . According to the assumption, and both form arithmetic progressions. Then, we have , implies that , which is clearly a contradiction.

Techniques

Pigeonhole principleCounting two ways