Ok, I know this is a little late, but here goes...
The whole 1/2 1/3 debate was bugging me, so I e-mailed her about this with links to a couple of different arguments (in addition to the thread it self). Here's a copy:
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When I first looked at the problem I quickly thought that it was just a simple case of conditional probability - and that the answer therefore must be 1/3.
However, after giving it some more thought, I'm would side with those who say that it is 1/2. [A friend] agrees.
The problem seems equivalent to: Bob flipped 2 coins. He showed me that one of them came up heads. What's the probability that the other coin is heads?
Essentially, the sex of the 2nd kid is independent of the sex of the
first. Knowing that one of the kids (the door opener) is a girl tells
us nothing about the sex of the second kid.
or to think about it another way. Suppose that there were 1000 doors to be opened. Opening a door and finding a girl there eliminates 500 of them. Of the 500 that are left, 250 of them will be GG. Therefore the probability is 1/2 -[Cool Math Profesor]
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Originally Posted by Astronouth7303
ok, on an on-line bulletin board I hang out at, some one posed the
following problem: Mr and Mrs Smith have two children. You ring at their door, and one of their two children, a girl opens the door. What is the probability that the other child is also a girl? (Warning: medium difficulty)
And now everyone is up in arms about whether it's 1/2 or 1/3! Can you enlighten me?
http://www.chiefdelphi.com/forums/sh...ad.php?t=29393
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She is a profesor at a local college.