I don't believe that two coins has any bearing on this situation.

We were told there was a 50% chance of something happening, we were told that one scenario had already happened. - a girl.

That still leaves a 50% chance.

Where did this extra coin come from?

Unless you count twins as one child

The available combinations given that one child is a girl is Girl-Boy, Girl-Girl, there is no way they can have boy-boy.

Boy-Girl is the same as Girl-Boy as they already have a girl.

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Hi CP!
That's why I told Dr Ron, that Woof had the right answer to the wrong question.

Statistics is a wonderful and indispensable tool in making sense of info, provided it is used the way it was meant to be used.
It has aquired a smelly and dubious reputation only because it is has been, and dare I say will be used in the future to prove things to be either true or false, when logic tells you otherwise.
Still let them prove what they want, as long WE use stats the right way!
Cheers

The coins example is no different to the boy/girl problem.
Where you are confused Chrome Prince is that the order IS important.
The sample space is in fact -

Boy/Boy
Boy/Girl
Girl/Boy
Girl/Girl

Where each outcome is a 1/4 or 25% chance of occuring.

So if the order is not spectific there is a 1/4(25%) chance of having 2 Boys, a 1/4(25%) chance of having 2 Girls and a 2/4(50%) chance of having a Boy/Girl (any order).


Now the problem is conditioned on the fact that there is "at least one girl" so the Boy/Boy scenario can be eliminated fom the sample space leaving -

Boy/Girl
Girl/Boy
Girl/Girl

Where each outcome is a 1/3 or 33% chance of occuring. Given that we have already singles out a Girl, we are left with a Boy in 2 out if the 3 possibilities. This leads to the 2/3 Boy and 1/3 Girl which happens to be the correct answer.

The hard part to comprehend is the wording of the question.

CP,

Get your facts right.

Your coin metaphor is wrong.

Woof clearly stated that both of the 2 children were already begat.

You are wrongly suggesting otherwise when you talk about the 2nd flip of a coin.

The next part of the stated problem, involved X - an information provider (not necessarily a parent KV!) who secretly ascertained the gender of one child. Then if that child was not a girl X then checked the gender of the other to see if that one was a girl.

All that X disclosed was that one of those 2 children was a girl.

You don't know how many childrens' relevant details X examined.

X didn't say that the younger was a girl. Nor that the older was a girl.

KV has wrongly assumed that the parents are informing you that one of the children is a girl.

But you are not making assumptions. Instead you have invented a contradictory scenario.

There is no flipping of a 2nd coin. Both coins have already been flipped.

Enough coins have been examined to truthfully disclose whether a head has occurred.

After I figured the problem through I then googled to see whether this was a classic problem.

Got it in one.

http://mathforum.org/dr.math/faq/faq.boy.girl.html

Meanwhile some of you couldn't even do the right thing and check whether you had read the problem correctly in the first place.

Imagine the response you can expect next time you ask a stats related question.

I tried hard to stay away from this. One billion sperm racing down the chute and the leader pulls up and lets the potential boy thru while saying "we have already been to the trough" Hard and fast the sperm has no prior memory etc This is a straight out 50/50 possible outcome.

At this point other factors arise. Most of these are statitical analysis of past results which have no direct bearing on this event. However people read their own interpretations from these results. The ratio of boys to girls is 50/50 and there is one girl already hence there is a 100% chance that this is a boy. Boy/girl and Girl/Girl is still only 50/50 because we are only dealing with the possible outcome so the factual portion (we already have a girl) is deleted leaving whether this one will be a boy or girl. Other factors may sutblely alter the odds ie a history of girls on both sides of the family,This area produces on average 75% girls or this is Amazonia and boys do not survive birth.
The only way to change it from 50/50 is to show that there is a greater proportion of one sex in the sperm count because nobody is going to say thatone sex swims faster than the other.

this is my only comment as i refuse to allow the quality of the forum to deteriorate.

Besides that I wish all a good day

jfc,i was just wondering if you get sea views from the mountain you live on.....

...........cheers....slowman...............

This is indeed the only correct answer. All other answers depend on the answer to this question.

KV

As far as I am concerned the above quote says it all. If there is a 50/50 chance of an event happening then what has happened before hand is irrelevent. JUst because the ratio of boys to girls born is 50/50 dosent mean to say a family cant have 2 girls and another family have 2boys. The ratio still stays the same.

can't wait for the follow up question woof.

Spot on DrRon,

I'm not wrong, and I don't understand if we've been told one is a girl already, why others cannot comprehend that Boy-Boy is an invalid combination.

Amazing.

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SLOWMAN

Did you know there are forums on the internet that are not gambling related?
Perhaps you could try a Google search using keywords like,
Forum,"bagging people","Low I.Q."

JFC is in fact one of my favourite contributors.
I don't quite understand his mathematics,but it sure sounds impressive.