A quiz consists of 10 multiple-choice questions with 5 choices for each question. Suppose a student is sure of the first 5 answers and has each of the last 5 questions narrowed to 3 of the possible 5 choices. If the student guesses among the narrowed choices on the last 5 questions, find the probability of
(a) Passing the quiz (getting at least 60%).
(b) Getting at least a B (at least 80%).