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Find the number of permutations of n distinct things taken r together in which 3 particular things must occur together.

$\begin{array}{1 1}(A)\;n-3C_{r-3}(r-2)!3!\\(B)\;nC_r\\(C)\;n-1C_{r-1}\\(D)\;n-2C_{r-3}\end{array} $

1 Answer

The number of permutations of n distinct things taken r together =$nC_r$
3 things together =$(n-3)C_{r-3}$
$\Rightarrow (n-3)C_{r-3}(r-2)!3!$
Hence (A) is the correct answer.
answered May 15, 2014 by sreemathi.v
 

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