In a multiple choice question Exam, a student either knows the answer or he guesses. There are m options available in each question. If he guesses the answer, he is correct with 1/m probability.
Suppose p is the probability that he knows the answer and 1-p is the probability he doesn't know.
What is the probability that he has given answer correctly?
This is a case of Total Probability.
P(correct) = P(he knowns answer and answer is correct) + P(he guess the answer and answer is correct)
= p(1) + (1-p)*(1/m)