Sunday, 3 May 2015

probability - What's the smallest $n$ so that it's more likely than not that someone among the $n$ people has the same birthday as you?



Suppose that the birthdays of different people in a group of $n$ people are independent, each equally likely to be on the $365$ possible days. (Pretend there's no such thing as a leap day.)



What's the smallest $n$ so that it's more likely than not that someone among the $n$ people has the same birthday as you? (You're not part of this group.)




At first, I get $n=23$ by using the complement rule to get the probability that someone has the same birthday as me $(P\{\text{same birthday}\} = 1 - P\{\text{different birthday}\})$ and use $e^{-x}=1-x$.





But I don't think the answer is $23$ since I am not part of the group.


Answer



HINT



The probability that none of the $n$ people have the same birthday as you is:




$$\big( \frac{364}{365} \big)^n$$



So, set this equal to $\frac{1}{2}$ and let WolframAlpha do its magic.



And as you alrady suspected, this problem is not like the infamous 'Birthday Problem'


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