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Question
Find x if `""^"n""P"_"r" = "x" ""^"n""C"_"r"`
Solution
`""^"n""P"_"r" = "x" ""^"n""C"_"r"`
x = `(""^"n""P"_"r")/(""^"n""C"_"r")`
= `(("n"!)/(("n" - "r")!))/(("n"!)/(("n" - "r")!"r"!))`
= `1/("r"!)`
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