SOLUTION: on a multiple choice test with 9 questions, each question has 4 possible answers, only one of which is correct. For students who guess at all answers, find the mean for the number

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Question 612040: on a multiple choice test with 9 questions, each question has 4 possible answers, only one of which is correct. For students who guess at all answers, find the mean for the number of correct answers.
I understand that the probability of getting a correct answer is 1/4 or .25 (my instructor likes all work in decimals). so to get the mean is it .25^9? also wouldn't I have to know how many students took the test to find out how many got them correct? I really don't understand this problem.
Answer by jim_thompson5910(35256) About Me  (Show Source):
You can put this solution on YOUR website!
This is a binomial distribution problem. So use the idea that the mean is np where n is the number of questions and p is the probability of getting any question correct.

Mean:

mu = n*p

mu = 9*(0.25)

mu = 2.25

So the mean number of correct answers is 2.25