Thursday, 22 February 2018

probability - Expected number of rolls when repeatedly rolling an n-sided die




Suppose I roll an n-sided die once. Now you repeatedly roll the die until you roll a number at least as large as I rolled. What is the expected number of rolls
you have to make?



I know the answer to this problem, but I'm curious about possible solutions people might post.


Answer



If you roll a k, then there are nk+1 possible numbers out of n that will be greater than or equal to k. This gives rise to a geometric distribution, and so the expected number of rolls required after rolling a k is nnk+1. Averaging over all k, the expected number of rolls will be E=1nnk=1nnk+1=Hn

where Hn is the nth harmonic series.


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