A multiple-choice examination consists of 80 questions, each having possible choices a, b, c, and d.

A multiple-choice examination consists of 80 questions, each having possible choices a, b, c, and d. Approximate the probability that a student will get at most 20 answers correct if he randomly guesses at each answer. (Note that, if he randomly guesses at each answer, then the probability that he gets any one answer correct is 0.25) Use the normal approximation to the binomial with acorrection for continuity.