On a multiple choice examination with three possible answers for each of the five questions, what is the probability that a candidate would get four or more correct answers just by guessing ?

In this question, we have the repeated correct answer guessing form the given multiple choice questions are Bernouli trials

Let us now assume, X represents the number of correct answers by guessing in the multiple choice set


Now, probability of getting a correct answer,


Thus,




Clearly, we have X is a binomial distribution where n = 5 and


∴



Hence, probability of guessing more than 4 correct answer


= P (X = 4) + P (X = 5)





9