Today's evil Bayesian math problem

From: Eliezer S. Yudkowsky (sentience@pobox.com)
Date: Mon Sep 08 2003 - 22:54:06 MDT

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    Suppose that a bowl has 5 red chips and 3 white chips. We sample chips
    from the bowl using the following procedure: On each round we draw a
    random chip, replace it, and then add another chip of the same color to
    the bowl. For example, if on the first round we happen to draw a red
    chip, there would then be 6 red chips and 3 white chips to draw from on
    the second round.

    Given that a white chip was drawn on the fourth round, what is the
    probability that a white chip was drawn on the second round?

    (This problem is extra bonus evil because it's so easy if you know the rules.)

    -- 
    Eliezer S. Yudkowsky                          http://singinst.org/
    Research Fellow, Singularity Institute for Artificial Intelligence
    


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