🚀 go-pugleaf

In article <1993Jan22.131719.36@janus.arc.ab.ca> morgan@arc.ab.ca (Sean Morgan)
writes:
>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?"
>-----------------+-------------------------------

(Some of my mail says "not again", and some says "please post a solution".
 Since you can't please all of the people all of the time...)

Two child families are uniformly distributed between MM,MF,FF,FM, in birth
order.  Again, all four possibilities are equally likely.  Stop here.  Make
sure you believe that.

Now, since at least one is F, the MM drops out, leaving MF, FF, FM, so that
there is only a one third chance that the sibling is a girl.
-----------------+-------------------------------
Sean Morgan      |  ALBERTA  6815 - 8 Street N.E.
                 | RESEARCH  Calgary, AB, Canada
morgan@arc.ab.ca |  COUNCIL  T2E 7H7