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Math Help - If X is a binomial random variable with expected value 6 & variance 2.4, find P(X=5).

  1. #1
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    If X is a binomial random variable with expected value 6 & variance 2.4, find P(X=5).

    Question:
    If X is a binomial random variable with expected value 6 and variance 2.4, find P(X=5).

    My try:
    I know this question is simple, but I think I'm making a silly mistake in my logic somehow. I started with E(x)=np and Var(X)=E(X^2) - (E(X))^2 so Var(X)= npq - (np)^2. I plugged in all the numbers, solved, and got q=6.4 ... yeah way off, q has to be between 0 and 1. So I stopped there since my implementation was obviously wrong.

    Thanks for any and all help.
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  2. #2
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    Hello,

    Note that Var(X) can be factored into npq, where q=1-p
    You know that np=E(X)=6.

    So 2.4=6q.
    And hence q=... ?
    then find p and n !
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