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Announced vs Final rank average (Posted on 2022-02-23) Difficulty: 3 of 5
In some televised sports, such as downhill skiing, individual competitors take turns for the best time on a course. After each competitor the announcers will give the current standing. So for example, the first person will always be announced as ranked 1 (though this will likely change), the second person will be announced as either 1 or 2, and so on.

By the end of the competition everyone's final rank will be the same or higher.

a) In an event with n competitors, what is the expected difference between the first announced and final rank of competitor that goes xth?

b) What is the expected average difference for all n competitors?

Note: the order of the competitors is random.

No Solution Yet Submitted by Jer    
Rating: 5.0000 (1 votes)

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Solution Solution Comment 2 of 2 |
For any competitor, their final rank could be anywhere from 1 to n; on average it will be (n+1)/2. 

The xth competitor's first announced rank could be anywhere from 1 to x; on average it will be (x+1)/2. 

So the expected difference between the first announced and final rank of the competitor that goes xth will be (n+1)/2 - (x+1)/2 = (n-x)/2.

The expected average difference for all n competitors would be the sum of (n-x)/2 from x = 1 to n, divided by n. So:

[(n-1)/2 + (n-2)/2 + (n-3)/2 + ... + (n-n)/2] / n

The numerator will be n*n - the nth triangle number, which simplifies to (n^2 - n)/4.  Dividing by n to get the average we get (n-1)/4.

  Posted by tomarken on 2022-02-23 10:31:21
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