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 counts the rolls made after the first one, what is ?
Short answer
. If the first roll is , each later roll succeeds with probability , so the wait is geometric with mean . Averaging over gives .
Adapted from QuantQuestions.io; question and answer rewritten by SelfMadeQuant.com.
See the hint and the full worked solution in the question bank (free account).
Open the worked solutionLessons behind this question
- Expectation, Mean, and Variance
- Counting and Combinatorics
- Conditional Expectation and Variance Revisited
Start with a free lesson: Expectation, Variance and Covariance is part of a free topic. Create a free account, no card needed, to read it in full with worked solutions.
Read it free