Expected Number of Trials until Success - GeeksforGeeks

Expected Number of Trials until Success - GeeksforGeeks
In a country, all families want a boy. They keep having babies till a boy is born. What is the expected ratio of boys and girls in the country?

If probability of success is p in every trial, then expected number of trials until success is 1/p

Let p be probability of having a baby boy.
Number of kids until a baby boy is born = 1/p 
                                        = 1/(1/2)
                                        = 2 
Since expected number of kids in a family is 2,
ratio of boys and girls is 50:50. 

http://www.quora.com/In-a-country-in-which-people-only-want-boys-every-family-continues-to-have-children-until-they-have-a-boy-If-they-have-a-girl-they-have-another-child-If-they-have-a-boy-they-stop-What-is-the-expected-proportion-of-boys-to-girls-in-the-country
http://mathoverflow.net/questions/17960/google-question-in-a-country-in-which-people-only-want-boys
http://en.wikipedia.org/wiki/Geometric_distribution
http://geeksquiz.com/puzzle-17-ratio-of-boys-and-girls-in-a-country-where-people-want-only-boys/
Coupon Collector Problem:
Suppose there are n types of coupons in a lottery and each lot contains one coupon (with probability 1 = n each). How many lots have to be bought (in expectation) until we have at least one coupon of each type.

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