2.18. A problem on a multiple-choice quiz is answered correctly with probability 0.9 if a student is prepared. An unprepared student guesses between 4 possible answers, so the probability of choosing the right answer is 1/4. Seventy-five percent of students prepare for the quiz. If Mr. X gives a correct answer to this problem, what is the chance that he did not prepare for the quiz?

2.18. A problem on a multiple-choice quiz is answered correctly with probability 0.9 if a student
is prepared. An unprepared student guesses between 4 possible answers, so the probability
of choosing the right answer is 1/4. Seventy-five percent of students prepare for the quiz. If
Mr. X gives a correct answer to this problem, what is the chance that he did not prepare
for the quiz?