Assume that 21 questions are answered by guessing. What is the probability of exactly 21 correct answers?

Answer 1

Assuming it is a multiple choice test and assuming that each question has N possible answer choices...

Probability of getting any particular question correct is 1/N

So binomial (21 choose 21) (1/N)^21 ((N-1)/N)^0 =

(1/N)^21