Rolling a d10 until the lcm passes 2000
Question
You roll a fair ten-sided die (faces to ) repeatedly and track the least common multiple of all the values rolled so far. Let the expected number of rolls until this lcm first exceeds be in lowest terms. Compute .
Short answer
. The lcm of is , and every proper divisor of it is at most , so you need the full : a , an , a and a multiple of ( or ). A coupon-collector style calculation (or a small Markov chain over lcm values) gives expected rolls.
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