🚀 go-pugleaf

In article <1993Jan22.131719.36@janus.arc.ab.ca>, morgan@arc.ab.ca (Sean
Morgan) wrote:
> 
> You meet the daughter of a friend of yours on the street.  You know that your
> friend has two children.  You think to yourself, "Ignoring the occurrence of
> twins, and sex-linked differences in birth rates and infant mortality, what are
> the odds of this girl's sibling also being a girl?"
> -----------------+-------------------------------
> Sean Morgan      |  ALBERTA  6815 - 8 Street N.E.
>                  | RESEARCH  Calgary, AB, Canada
> morgan@arc.ab.ca |  COUNCIL  T2E 7H7


I don't think you asked that quite right.  It's supposed to be, "A man
has two children.  One of them is a boy.  What are the odds that the
other one is also a boy?"
The answer, which surprises most people, is 1/3.  There are four
possibilities: boy/boy, boy/girl, girl/boy, and girl/girl, all equally
likely.  Since one child is a boy, the last possibility is eliminated.
That leaves two out of three cases where the other child is a girl.

However, meeting one of the children on the street changes the odds,
because if they are both girls you'll be twice as likely to meet one
of them.  That is, after meeting the girl you do know that there are
only three possibilties, and that they are not both boys, but the
three possibilities are not equally likely.  Assuming, that is, that
your odds off meeting someone of type X on the street depend on how
populous people of type X are.


Bob Hearn
Spartacus Software

Opinions expressed here are those of my empolyer, since that's me.