Rolls needed given a six precedes any five
Question
A fair die is rolled repeatedly until the first shows up. Conditional on no having appeared before that first , what is the expected total number of rolls?
Short answer
. Let be the first roll showing a 5 or a 6; is geometric with success probability , and whether that roll is a 5 or a 6 is independent of . Conditioning on it being a 6 makes the total number of rolls equal to , whose mean is still . (Simply deleting 5s from the die and answering 5 is a common mistake.)
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Open the worked solutionLessons behind this question
- Conditional Probability, Total Probability, and Bayes' Rule
- Counting and Combinatorics
- Expectation, Mean, and Variance
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