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Math Help - [SOLVED] Expectation and Variance

  1. #1
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    Joined
    Sep 2008
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    [SOLVED] Expectation and Variance

    In a multiple choice test there are 10 queastion and each had 4 choices.

    If a student guesses at each of the answer, what is the prob of getting none corrects? /// I just want to prove my answer:

    p(X=0)
    = 10c0
    = 10!/ 0!(10-0)!
    = 10!/10!
    =1

    b.) More than 7 corrects:

    10c8 * 10c9 * 10c10

    =10!/ 8!(10-8)! * 10!/ 9!(10-9)! * 10!/ 10!(10-10)!
    =45 * 10 * 1
    =450

    *Just wanna know if they are correct!!!tnx
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  2. #2
    Member
    Joined
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    Singapore
    Posts
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    I don't think your approach is right. Here is what i would do.

    p( getting correct answer ) = 0.25
    q = 1 - 0.25 = 0.75
    P( of getting none correct ) = 10C0 x (0.25)^0 x (0.75)^10

    b.) More than 7 corrects
    p( more than 7 corrects ) = P(x=8) + P(x=9) + P(x=10)
    = 10C8 x (0.25)^8 x (0.75)^2 + 10C9 x (0.25)^9 x (0.75)^1 + 10C10 x (0.25)^10 x (0.75)^0


    Quote Originally Posted by geeko View Post
    In a multiple choice test there are 10 queastion and each had 4 choices.

    If a student guesses at each of the answer, what is the prob of getting none corrects? /// I just want to prove my answer:

    p(X=0)
    = 10c0
    = 10!/ 0!(10-0)!
    = 10!/10!
    =1

    b.) More than 7 corrects:

    10c8 * 10c9 * 10c10

    =10!/ 8!(10-8)! * 10!/ 9!(10-9)! * 10!/ 10!(10-10)!
    =45 * 10 * 1
    =450

    *Just wanna know if they are correct!!!tnx
    Last edited by tester85; December 10th 2008 at 12:41 PM.
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