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In how many of the permutations of n thi...

In how many of the permutations of `n` thing taken `r` at a time will three given things occur?

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According to the condition of the question, three things are already selected. We have to select (r-3) more things from remaining (n-3) things and then we have to arrange r things.
So, required number of permutations `.((n-3))C_(r-3)xxr!`
`=((n-3)!r!)/((r-3)!(n-r)!)`
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