[qwnu]

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You need to use bayes' theorem to find the probability that you selected the two blue marbles from a given hat, and then from there it is straight forward to calculate the chance that you grab a red marble from the other hat.

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i dont think any of us have heard of baye's theorem, and i dont get how the first picking affects the second

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Once you pick a hat, all you know is that there's a 50% chance you have hat A, and a 50% chance you have hat B.

But after you pick out two blue marbles from your first hat, you have some additional information. It should be intuitively obvious that it is now more likely that you have hat B. Bayes Theorem allows you to calculate this likelihood precisely.