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Each of the arrangement which can be made by taking some or all of a number of thing is called a permutation
n number of things can be selected in n! ways
P_{r}^n is Number of permuatations of n different things taken r at a time
P_{r}^n= \frac{n!}{(n-r)!}=n(n-1)(n-2) ....(n-r+1)
Post ur questions it depends mainly on question
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Formula:   is the answer
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