It’s so counter-intuitive my head is going to explode.
Here’s the paradox for the uninitiated:If I say, “I have 2 kids, at least one of which is a girl.” What is the probability that my other kid is a girl? The answer is 33.33%.
>*Intuitively, most of us would think the answer is 50%. But it isn’t. I implore you to read more about the problem.*
Then, if I say, “I have 2 kids, at least one of which is a girl, whose name is Julie.” What is the probability that my other kid is a girl? The answer is 50%.
>*The bewildering thing is the elephant in the room. Obviously. How does giving her a name change the probability?*
>
>*Apparently, if I said, “I have 2 kids, at least one of which is a girl, whose name is …” The probability that the other kid is a girl* ***IS STILL 33.33%.*** *Until the name is uttered, the probability remains 33.33%. Mind-boggling.*
And now, if I say, “I have 2 kids, at least one of which is a girl, who was born on Tuesday.” What is the probability that my other kid is a girl? The answer is 13/27.
>*I give up.*
Can someone explain this brain-melting paradox to me, please?
In: 4
For the scenario “I have 2 kids, at least one of which is a girl, whose name is Julie”, start again with the basic possibilities:
Boy/Boy. Boy/Girl. Girl/Boy. Girl/Girl.
Now, *the parent knows* which of the children (the older or the younger) is Julie. Let’s say that Julie is the younger child.
So now the possibilities become:
Boy/Boy. Boy/Julie. Girl/Boy. Girl/Julie.
There are only two possibilities in which Julie is the younger child, and of those possibilities the chance of both children being girls is 1 in 2, or 50%.
We could also have assumed that Julie is the older child, and the result is the same.
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