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December 5th, 2016, 11:28 AM  #1  
Newbie Joined: Dec 2016 From: kljlçjkçlkjç Posts: 2 Thanks: 0  Probability to be born on a specific day
Problem: Quote:
I don't understand the two formulas (for getting 7.9% and for 70%). Could someone explain me, please?  
December 5th, 2016, 02:12 PM  #2 
Global Moderator Joined: May 2007 Posts: 6,438 Thanks: 562 
Prob. not a specified date (Sep. 16) for one person is 364/365. Birthdays are assumed independent, so prob. that n people's birthday is not on specified date is $\displaystyle (\frac{364}{365})^n$, based on general rule that the probability of n independent events all happening is the product of the individual probabilities. For the second question: The probability that two people have different birthdays is 364/365. For 3 people it is (364/365)(363/365). etc. 
December 6th, 2016, 02:47 AM  #3 
Newbie Joined: Dec 2016 From: kljlçjkçlkjç Posts: 2 Thanks: 0 
ty very much mathman. the wikipedia link has a reference explaining the formula that I had not seen: http://mathforum.org/dr.math/faq/faq.birthdayprob.html 

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born, day, probability, specific 
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