Problem: There are two cards; one is red on both sides and the other is red on one side and blue on the other. The cards have the same probability () of being chosen, and one is chosen and placed on the table. If the upper-side of the card on the table is red, then the probability that the under-side is also red, is
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Solution:
Let the sides of the first card both red) be numbered and . Let the red and blue sides of the second card be numbered and respectively. On draw, any one of the four sides could come up. But side does not, so or must. Of these, three, two undersides are red and one blue so that the probability of red is two out of three or .