Ordering three second-moment quantities for dice
Question
Throw a fair die five times independently, with results , and let be their median (the third smallest). Consider three quantities: (a) the expected product of the first two results, ; (b) the expected square of the first result, ; (c) the expected square of the median, . Order them from largest to smallest and report the order as a three-digit number using , , (so would mean ).
Short answer
. Independence gives (a) , and (b) . The median has mean 3.5 but is more concentrated than a single roll, so its second moment sits between the two: (c) . Thus .
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Open the worked solutionLessons behind this question
- Random Variables and Probability Mass Functions
- Counting and Combinatorics
- Expectation, Mean, and Variance
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