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jmc

counting and probability senior

Problem

If 2 cards from a standard deck are selected randomly, what is the probability that either two kings or at least 1 ace occurs? (There are 4 Aces, 4 kings and 52 total cards in a standard deck.)
Solution
We clearly can't have both two kings and at least 1 ace, so we have two exclusive cases to evaluate separately.

Case 1: Two kings. The probability that two kings are drawn is .

Case 2: At least 1 ace. We can break this into two cases:

Subcase 2A: Exactly 1 ace. We can choose the ace first with probability , and we can choose the ace last with probablity . So, the total probability of getting exactly one ace is .

Subcase 2B: 2 aces. The probability of this occurring is the same as that of two kings, .

So, the total probability for Case 2 is .

Adding this to our probability for Case 1, we have .
Final answer
\frac{2}{13}