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Math Help - Birthday Problem

  1. #1
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    Birthday Problem

    Hi all,

    I am new to this forum so please excuse me if I am posting this in the wrong section. I have, what I think is, a basic statistics problem:

    Let X equal the number of people selected at random that you must ask in order to
    find someone with the same birthday as yours. Assuming each day of the year is
    equally likely (and ignoring February 29).
    (a) What is the p.m.f. of X?
    (b) Find P(X > 300).

    I have found that the p.m.f = (1/365)(364/365)^(x-1)

    I do not understand how to find P(X>300)

    Help would be appreciated
    Thank you
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  2. #2
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    Quote Originally Posted by Buttons225 View Post
    Hi all,

    I am new to this forum so please excuse me if I am posting this in the wrong section. I have, what I think is, a basic statistics problem:

    Let X equal the number of people selected at random that you must ask in order to
    find someone with the same birthday as yours. Assuming each day of the year is
    equally likely (and ignoring February 29).
    (a) What is the p.m.f. of X?
    (b) Find P(X > 300).

    I have found that the p.m.f = (1/365)(364/365)^(x-1)

    I do not understand how to find P(X>300)

    Help would be appreciated
    Thank you
    Pr(X > 300) = Pr(X = 301) + Pr(X = 302) + .... + Pr(X = 365).
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