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The blue urn contains two blue balls and one red ball

There are two urns: a red one and a blue one

The red urn contains two red balls and one blue ball

?

However, you don't know which urn is used...

And both urns are as likely

You will see the color of two of the three balls...

it has to be the blue urn, because the red urn doesn't contain two blue balls

If both balls are blue...

it has to be the red urn, because the blue urn doesn't contain two red balls

And if both balls are red...

the urn can be the red or the blue one, equally likely.

But if one ball is blue and one is red...

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