SOLUTION: in a group of 4 friends, what is the probabilty that a. randomly selected person will have a birthday in july b. 4 of them have different birthday c. at least 2 have the same b

(a)  Assuming that there is no twins among the friends,  P =  = .



(b)  Assume the the year has 365 days and there in NO birthday on Feb.,29.


     First of the 4 friends has his birthday at some day;

     the probability that 2-nd friend has his birthday at different day is  ;

     the probability that 3-rd friend has his birthday at different day than the first two is  ;

     the probability that 4-th friend has his birthday at different day than the first three is  ;

     The answer to this question is  P =  = 0.983644   (approximately).



(c)  Probability (c)  is the complement to probability  (b)


         P = 1 - 0.983644 = 0.016356.