You have 5 2 ! (yes fifty two factorial) decks of standard playing cards each shuffled independently.
What is the probability that at least one of them is perfectly sorted, that is the Ace through king of spades, followed by the Ace through king of diamonds, then the Ace through king of clubs, and finally the Ace through king of hearts?
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For N cards, the probability of a given distribution is N ! 1
For N combinations the probability of having none of them correct is ( N N − 1 ) N
Therefore, the probability that at least one will be sorted is 1 − ( N N − 1 ) N
For large N like, for example N = 5 2 ! this goes 1 − e 1 = 0 . 6 3
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First, we calculate the probability (P) that none of the decks will be sorted as required.
Probability that a deck is sorted in a specific arrangement = 5 2 ! 1
Hence, probability that all decks are not sorted according to that arrangement = ( 1 − 5 2 ! 1 ) 5 2 ! = P
Now, since 5 2 ! is an astronomically massive number, we can consider it to tend to ∞ . Hence,
P = x → ∞ lim ( 1 − x 1 ) x = e 1 ≈ 0 . 3 6 7
The required probability is 1 − P = 0 . 6 3 2