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Math Help - binomial

  1. #1
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    binomial

    sorry, im really dumb.. but here's the question

    A person researching business-to-business communications found 90% of people used email as their primary means of communication. If there were 20 B2B operators contacted for a future research study focused solely on email communication, what is the probability that none of the 20 will use email in B2B as their primary form of communication?


    (a)  0.1^{20}
    (b) 1 - 0.1^{20}
    c)  0.9^{20}
    d) 1 - 0.9^{20}

    i know the answer is a

    but can someone just explain to me, why didnt they sub the values in the binomial formula? coz i thought that you have to use the full formula when calculating the binomial, but then why did they only use a part of it?
    Last edited by gconfused; September 22nd 2009 at 05:27 PM.
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  2. #2
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    Quote Originally Posted by gconfused View Post
    sorry, im really dumb.. but here's the question

    A person researching business-to-business communications found 90% of people used email as their primary means of communication. If there were 20 B2B operators contacted for a future research study focused solely on email communication, what is the probability that none of the 20 will use email in B2B as their primary form of communication?

    (a)  0.1^{20}
    (b) 1 - 0.1^{20}
    c)  0.9^{20}
    d) 1 - 0.9^{20}

    i know the answer is a

    but can someone just explain to me, why didnt they sub the values in the binomial formula? coz i thought that you have to use the full formula when calculating the binomial, but then why did they only use a part of it?
    It helps to define the random variable and hence get things clear in your head:

    Let X be the random variable number of people that will use email in B2B as their primary form of communication.

    X ~ Binomial(n = 20, p = 0.9)

    \Pr(X = 0) = {20 \choose 0} (0.9)^0 (1 - 0.9)^{20 - 0} = 1 \times 1 \times 0.1^{20}.
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