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Math Help - Normal distribution approximation

  1. #1
    Junior Member
    Joined
    May 2010
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    26

    Normal distribution approximation

    A multiple choice test consists of 50 questions and each question has four answers from which to choose. If a student guesses every answer, what is the probabilty that the student
    b) will get more than 10 questions correct?

    here is what i did. I got close, so it could just be the book??
    p=1/4 = .25 np=(.25)(50) = 12.5 therefore np is greater than 5
    q=3/4=.75 nq=(.75)(50)= 37.5 therefore nq is great than 5

    Since np and nq are both greater than five we can use the formula Standard deviation =squareroot(npq) (sorry i don't know the computer way to do that)

    standard deviation = root((50)(.25)(.75))
    =3.061862178

    p(11 or more) because you want to know the probablitiy of getting more than 10 questions correct so 10 is not included

    np=mean=12.5 this is the thing im not sure about .. but i found it in my notes so i went with it.

    z=(11-12.5)/3.061862178
    =-0.49 area to the left = 0.3121
    we want the area to the right
    1-0.3121 = 0.6879
    so 68.79%

    the answer in the back of the book is 74.32%
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  2. #2
    Junior Member
    Joined
    May 2010
    Posts
    26
    haha. nevermind. i figured out the answer to this one
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