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There are three urns.
- The first contains 2 white and 4 black cards
- The second contains 8 white and four black cards
- The third contains 1 white and 3 black card.
If one card is selected from each urn, find the probability that exactly 2 white cards are drawn"
What Plato is getting at is suppose the two white cards come from urns 1 and 2, repectively. Therefore, the third card, black, must come from urn 3.
Now, do the same for the other two cases and add them up. Supoose they come from urns 1 and 3 and urns 2 and 3