MathDB
Expected rank of elimination tournament

Source: Simon Marais Mathematics Competition 2023 Paper B Problem 2

October 16, 2023
probabilityexpected valuecombinatorics

Problem Statement

There are 256256 players in a tennis tournament who are ranked from 11 to 256256, with 11 corresponding to the highest rank and 256256 corresponding to the lowest rank. When two players play a match in the tournament, the player whose rank is higher wins the match with probability 35\frac{3}{5}.
In each round of the tournament, the player with the highest rank plays against the player with the second highest rank, the player with the third highest rank plays against the player with the fourth highest rank, and so on. At the end of the round, the players who win proceed to the next round and the players who lose exit the tournament. After eight rounds, there is one player remaining and they are declared the winner.
Determine the expected value of the rank of the winner.