試題詳解

117. In each of the months June, July, and August, the number of accidents occurring in that month is modeled by a Poisson random variable with mean 1. In each of the other 9 months of the year, the number of accidents occurring is modeled by a Poisson random variable with mean 0.5. Assume that these 12 random variables are mutually independent.
Calculate the probability that exactly two accidents occur in July through November.
(A) 0.084
(B) 0.185
(C) 0.251
(D) 0.257
(E) 0.271