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Math Help - Simple Binomial Distribution question (need clarification)

  1. #1
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    Simple Binomial Distribution question (need clarification)

    Hi
    the following question i have done, but my answer is incorrect.

    A company receives 70% of its orders over the internet. On a particular day it receives 32 independent orders. Calculate the probability that between 20 and 30 (both inclusive) of the orders are received over the internet?

    This is what i have done

    Pr(20 <= X <= 30) = Pr(X <=30) - Pr(X <= 20) = binomcdf(32,.7,30)-binomcdf(32,.7,20) = .7715

    However when i done binomcdf(32,.7,30)-binomcdf(32,.7,19) = 0.864 which is the correct answer, but don't understand how this is correct.

    according to the question if "both inclusive" shouldn't it be Pr(20 <= X <= 30) ?

    P.S
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  2. #2
    Grand Panjandrum
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    Re: Simple Binomial Distribution question (need clarification)

    Quote Originally Posted by Paymemoney View Post
    Hi
    the following question i have done, but my answer is incorrect.

    A company receives 70% of its orders over the internet. On a particular day it receives 32 independent orders. Calculate the probability that between 20 and 30 (both inclusive) of the orders are received over the internet?

    This is what i have done

    Pr(20 <= X <= 30) = Pr(X <=30) - Pr(X <= 20) = binomcdf(32,.7,30)-binomcdf(32,.7,20) = .7715

    However when i done binomcdf(32,.7,30)-binomcdf(32,.7,19) = 0.864 which is the correct answer, but don't understand how this is correct.

    according to the question if "both inclusive" shouldn't it be Pr(20 <= X <= 30) ?

    P.S
    You want:

    Pr(20 <= X <= 30) = P(X=30)+...+P(X=20)

    ......... =[P(X=30)+..+P(X=0)] - [P(X=19)+...+P(X=0)]

    ......... = Pr(X <=30) - Pr(X <= 19)

    CB
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  3. #3
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    Re: Simple Binomial Distribution question (need clarification)

    but why is it from 19->0 shouldn't it be 20->0 if your saying including 20?
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  4. #4
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    Re: Simple Binomial Distribution question (need clarification)

    Quote Originally Posted by Paymemoney View Post
    but why is it from 19->0 shouldn't it be 20->0 if your saying including 20?
    The complement of P(X\ge 20) is 1-P(X< 20)=1-P(X\le 19)~.
    You want to include 20 and exclude 19 and below.
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  5. #5
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    Re: Simple Binomial Distribution question (need clarification)

    i understand now thanks
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