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Rolls needed to match or beat the opening roll

Question

A fair six-sided die is rolled once and the result is noted. The die is then rolled again and again, stopping as soon as a roll is greater than or equal to that first result. If NN counts the rolls made after the first one, what is E[N]\mathbb{E}[N]?

Short answer

49/2049/20. If the first roll is kk, each later roll succeeds with probability (7−k)/6(7-k)/6, so the wait is geometric with mean 6/(7−k)6/(7-k). Averaging over kk gives 16∑k=1667−k=1+12+⋯+16=4920\tfrac16\sum_{k=1}^6 \tfrac{6}{7-k}=1+\tfrac12+\dots+\tfrac16=\tfrac{49}{20}.

Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.

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