🚀 go-pugleaf

>>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?"
>>-----------------+-------------------------------
>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.

But you meet the F on the street. So MF and FM are the same in this case
(but not the way the problem is usually stated). I say 50%.