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My brother and I were talking shortly after he won a hand in which his 4 of a kind beat someones full house. The board read 4 4 k k 5. My brother had a pair of 4's in the hole, his opponent had KQ.

After the hand I hypothetically asked him "would if a king came on the river?". He then asked "what would be the odds of that?" We then went tried pathetically to guess the odds with little to no knowledge about the math.

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Actually, that question is easier. There is 1 King left in the deck. The probability that the 4th King would come on the River would be 1/44 (44 cards left unseen) or 2.27%.