Who is the murderer? (§8.2)
A murder is committed. The perpetrator is either one or the other of the two persons X and Y. Both persons are on the run from authorities, and after an initial investigation, both fugitives appear equally likely to be the perpetrator. Further investigation reveals that the actual perpetrator has blood type A. Ten percent of the population belongs to the group having this blood type. Additional inquiry reveals that person X has blood type A, but offers no information concerning the blood type of person Y . What is your guess for the probability that person X is the perpetrator?