Posted by **please help me!** on Tuesday, March 23, 2010 at 2:21pm.

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?

- statistics -
**Reiny**, Tuesday, March 23, 2010 at 5:04pm
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

- statistics -
**gio**, Friday, February 18, 2011 at 10:09pm
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)

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