Children

[Buzzie]

A couple have two children (and they are a normal couple with a 50:50 chance of having a girl or a boy). You are intorduced to one of their children who is a girl. What is the probability that the other child is also a girl? There is a QI claxon waiting for the unwary.....

[Gizmonster]

It is you that is looking at it from the wrong angle.

I can't keep stressing this enough - the doctor doesn't just look at one of the children - he looks at both, then he makes his statement that at least one of them is a girl.

You agree that before the doctor looks at the children the following are all equally likely??
bb
bg
gb
gg

You agree with this so far yeah ??

Now when the doctor looks AT BOTH CHILDREN and sees a girl, the bb can be eliminated, therefore, there are only 3 possible pairs of children:

boy born first, girl born 2nd
girl born first, boy born 2nd
girl born first, girl born 2nd

I know you'll probably disagree with the above statement - but the jump from 4 possibilities to 3 is not that difficult to grasp, surely ??

It is the pairs of children that we are concerned with, not just the one girl!!
I suggest you read them links that I posted as I'm obviously not gonna get through to you - all the evidence is in them links - if you wanna ignore the evidence, that's up to you .....

[Buzzie]

Thats what I've being trying to stress GM but perhaps not as clearly as you - that you need to take the two facts together & not in isolation, ie there are (already) two children AND one of them is a girl!!
I really didn't think this riddle was that complicated to warrant 41 replies ;-)