statistics

A multiple choice test has 20 questions with each having 4 possible answers with one correct. Assume a student randomly guesses the answer to every question.

1) What is the probability of getting exactly 9 correct answers?

2)What is the probability of getting less than 9 correct answers?

asked by please help me!
  1. prob of R(ight) = 4/20 = 1/5
    prob of W(rong) = 4/5

    1) to have exactly 9R and 11W
    = C(20,9)((1/5)^9(4/5)^11
    = ....

    ( I got .00722)

    2) to get the prob of less than 9 you will have to do
    prob(0R) + prob(1R) + prob(2R) + .. + prob(8R)

    I will do prob(R4)
    = C(20,4)(1/4)^4(4/5)^16 = .21819

    Follow the same pattern for the other calculations

    posted by Reiny
  2. a test consists of 25 multiple choice questions. Each has 5 possible answers, which only one is correct. If a student guesses on each question, find the following:
    a) prob that he will guess all of them right
    b)prob that he will guess at most 12 right
    c) prob that he will guess at least 1 right
    d) mean and stndrd. deviation of the number of correct answers
    e)estimate the prob of the number of correct answers that will fall within the limits -----> miu +or- 2(stand.dev)

    posted by gio
  3. Suppose a student takes a 20 question multiple-choice test and guesses at every answer. If there are exactly four choices on each question, what is the probability the student passes the test? (Assume passing means the student gets at least 60% of the questions correct.)

    posted by Glenda
  4. 0999

    posted by Anonymous

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