50%, 1/2, half, whatever you want to call it.
People, for once and all, something that has already happened (in this case the sex of one child has been announced) CAN NOT EVER influence the probability of something happening in the future.
As we know one child is a girl, the options in this case are thus:
Girl + Girl
Girl + Boy
As the first child being a girl DOES NOT influence the probability of the 2nd child's sex, and the quoted probability of any further child being 50/50, then this is the probability of there being a boy and a girl.
funkynick, how the hell did you get 66%? Did you pay attention to maths lessons in school?
jarl, same comment as for funkynick.
God why don't people understand probability! ARGH!!! I have presented to some very high level people in big corporations, on large salaries several times talking about probability, and almost without exception they NEVER seem to understand that the fact that an event has happened (ie. flipping a coin, the sex of a child etc etc.) DOES NOT influence any upcoming events!
You flip a coin 4 times in a row, you get 4 straight heads. What's the probability of getting a head on the 5th attempt?
Answer is it's 50%, unless you have a 2 headed coin, in which case it would be 100%
Though don't confuse this with working out the probability of flipping 5 straight heads (on a coin with a heads and a tails on it) before any flipping occurs. This would of course be 1/2 x 1/2 x 1/2 x 1/2 x 1/2 = 1/32 (or 0.03125 for those that only do decimals).
Oh, and as for the plane on a conveyor, the wheels on a plane have nothing to do with its ability to take off or not. Air speed over the wings is what matters!