Two of three prisoner A, B and C will be executed, A asks the name of one other than A himself who will be executed. Jailer says that it is B. Merely by asking the question, A reduced the probability that he will be executed from 2/3 to 1/2, regardless of which answer.
What is wrong with the reasoning? I think the key is jailer answer the question by excluding the situation of A. So the answer just reduced the probability that C will die. A had better ask a question like if B will be executed but I want to ask the accurate reasoning by some formula.