# Birthday Problem

• Jul 30th 2010, 06:31 PM
Buttons225
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
• Jul 30th 2010, 11:10 PM
mr fantastic
Quote:

Originally Posted by Buttons225
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).