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Math Help - [SOLVED] binomial distribution problem

  1. #1
    chaosx
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    [SOLVED] binomial distribution problem

    n=7,r=3,p=0.25
    Pr= 7!/3!(7-3)!=1(0.25)^3(1-0.25)^4 =0.004943848.
    Why is this answer wrong ?
    The correct answer is 0.173035
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by chaosx View Post
    n=7,r=3,p=0.25
    Pr= 7!/3!(7-3)!=1(0.25)^3(1-0.25)^4 =0.004943848.
    Why is this answer wrong ?
    The correct answer is 0.173035
    For the Binomial distribution:

    Pr(X=r)={n \choose r}p^r(1-p)^{n-r}

    so in this case:

    Pr(X=3)={7 \choose 3}\,0.25^3\,0.75^4=\frac{7!}{3!\,4!}\,0.25^3\,0.75  ^4=0.173035

    RonL
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