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 Winner moves (Posted on 2018-08-20)
You are one of ten friends who are going to play a few rounds of a simple two-player game of chance. Each player has a 50% chance of winning the game each round.

There are 5 playing squares on the floor, arranged in a circle.

In the beginning, two people go to each square. Each round the people at a square play the game and determine the winner. A round ends when the winner walks clockwise to the next square. The loser stays where they are.

After 10 rounds, what is the probability you are on the square you started on?

 No Solution Yet Submitted by Jer No Rating

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 Let me count the ways (spoiler) | Comment 1 of 2
You are on the same square if you win 0 or 5 or 10 games.

There is 1 way to win 0 games, and 1 way to win 10 games.
The number of ways of winning exactly 5 games = C(10,5) =
10!/5!5! = 252

Requested probability = 254 / 2^10 = 127/2^9 = 127/512 ~ 24.8%

 Posted by Steve Herman on 2018-08-20 08:57:51

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