# Probability of given onto Function [duplicate]

Let $X = \{1, 2, 3, \ldots , 25\}$. If a student selects a function randomly from the set of all functions from $X$ onto $X$, then what is the probability that the selected function maps prime numbers to prime numbers?

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## marked as duplicate by drhab, 5xum, Norbert, Davide Giraudo, egregMay 23 '14 at 10:10

Hint: a function from $X$ onto $X$ (where $X$ is a finite set) is a permutation of $X$. If $A$ and $B$ are the sets of primes and non-primes in $X$, a permutation of $X$ that maps $A$ onto $A$ (and therefore $B$ onto $B$) corresponds to an arbitrary permutation of $A$ and an arbitrary permutation of $B$. Now, how many are there?