On a multiple choice test with 19 questions, each question has four possible answers, one of which is correct. For students who guess at all answers how do you find the mean for the number of correct answers?
The mean (or expected number) of correct answers is 4.75.
Each question has four possible answers. If guessing at random, there is a one in four chance, i.e. 25% chance, of getting a particular question right. This is the same probability for each of the 19 questions.
Therefore, if there is a 25% chance of getting any given question right, and there are 19 questions, then the total number of questions you will expect to get right is 25% * 19 = 4.75.