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408 probability/hospital.p




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This article is from the Puzzles FAQ, by Chris Cole chris@questrel.questrel.com and Matthew Daly mwdaly@pobox.com with numerous contributions by others.

408 probability/hospital.p


A town has two hospitals, one big and one small. Every day the big
hospital delivers 1000 babies and the small hospital delivers 100
babies. There's a 50/50 chance of male or female on each birth.
Which hospital has a better chance of having the same number of boys
as girls?

probability/hospital.s

The small one. If there are 2N babies born, then the probability of an
even split is

(2N choose N) / (2 ** 2N) ,

where (2N choose N) = (2N)! / (N! * N!) .

This is a DECREASING function.

If there are two babies born, then the probability of a split is 1/2
(just have the second baby be different from the first). With 2N
babies, If there is a N,N-1 split in the first 2N-1, then there is a
1/2 chance of the last baby making it an even split. Otherwise there
can be no even split. Therefore the probability is less than 1/2
overall for an even split.

As N goes to infinity the probability of an even split approaches zero
(although it is still the most likely event).

 

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