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Probability Question (Binomial RV)

Shazkar ShadowstormShazkar Shadowstorm Registered User regular
edited January 2007 in Help / Advice Forum
I'm a tad confused by the book and what I'm supposed to be doing for this one problem..
It says: let x be a binomial random variable with E[X]=7 and Var(X) =2.1

and then I need to find a) P{X=4} and P{X>12}

I know I need to use the binomial prob mass density function.. I'm just not sure how everything fits into it. The Var(X) part is what is confusing me specifically.

EDIT: Would I be correct in saying that E[X] = p and Var(X) = p(1-p) ? Err...

EDIT2: Can someone check if the first part would be right if I had it be (10 choose 4)(.7^6)(.3^4).. still dont know how to do part b though.

Shazkar Shadowstorm on


  • HerschelHerschel Registered User regular
    edited January 2007
    Binomial distribution has E(X)=np and Var(X)=(1-p)np
    P(X=x) = (n choose x) * ((1-p)^(n-x)) * p^x

    You know E(X) and Var(X), so you can solve for n and p.

    Then, you know the probability function, so you can get P(X=x) and P(X>x) :)

    Herschel on
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