Browse · MATH
Printjmc
counting and probability senior
Problem
Bob rolls a fair six-sided die each morning. If Bob rolls a composite number, he eats sweetened cereal. If he rolls a prime number, he eats unsweetened cereal. If he rolls a 1, then he rolls again. In a non-leap year, what is the expected number of times Bob will roll his die?
Solution
Let the expected value of the number of times Bob rolls his die on a single day be . When Bob rolls his die, there is a chance that he will stop rolling after one roll, and a chance that he will have to start over. In the second case, since his first roll had no effect on the outcome, Bob will on average roll his die more times, for a total of times that day. Therefore, we know that , or that . Over 365 days, Bob will roll his die an average of times total.
Final answer
438