Permutations And Combinations Question 125

Question: The numbers of permutations of $ n $ things taken $ r $ at a time, when $ p $ things are always included, is

Options:

A) $ ^{n}C _{r}\ p\ ! $

B) $ ^{n-p}C _{r}\ r\ ! $

C) $ ^{n-p}{C _{r-p}}\ r\ ! $

D) None of these

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Answer:

Correct Answer: C

Solution:

  • Since number of selections are $ ^{n-p}{C _{r-p}}. $ Therefore the arrangement of r things can be done in r ! ways. Hence the total permutations are $ ^{n-p}{C _{r-p}}r! $