Probability: Die roll until first number

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SUMMARY

The discussion focuses on calculating the probability that the number of rolls, X, until the first occurrence of rolling a '5' on a fair die is divisible by 3. The approach involves understanding the geometric distribution of die rolls, where P(X=n) can be derived. Specifically, the probability that X is divisible by 3 is expressed as the sum of probabilities P(X=3k) for integer k. This requires deriving the general formula for P(X=n) based on the outcomes of each roll.

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Homework Statement



Roll a fair die until the first time you roll a '5' then stop. Let X be the number of times you rolled the die. What is the probability that X is divisible by 3?

Homework Equations



E[X] ?

The Attempt at a Solution



I honestly have no idea how to even approach this sort of thing. My first thought was either a bionomial or geometric series, but that could possibly be endless.

any sort of hints or suggestions would be great.
 
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Let's first start by figuring out the probabilities involved.

Can you find out P\{X=1\}? (that is: the probability your first throw is a 5). What about P\{X=2\}? (the probability that your first throw is not a five, but your second throw is). Can you find a general formula for P\{X=n\}?
 
Find P(X = x). X is divisible by 3 if and only if X = 3k for some integer k. Therefore, the probability that X is divisible by 3 is given by \sum_{k=1}^{\infty} P(X = 3k).
 

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