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{same birthday}=1P{different birthday}) and use ex=1x.





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:




(364365)n



So, set this equal to 12 and let WolframAlpha do its magic.



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


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