If ^n{Pᵣ} = 840,{,^n}{Cᵣ} = 35, then n is equal to

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6.Permutation and Combination

easy

If $^n{P_r} = 840,{\,^n}{C_r} = 35,$ then $n$ is equal to

A

$1$

B

$3$

C

$5$

D

$7$

Solution

(d) $\frac{{^n{P_r}}}{{^n{C_r}}} = 24 \Rightarrow r\;! = 24 \Rightarrow r = 4$

$\therefore $$^n{C_4} = 35 \Rightarrow n = 7$.

Standard 11

Mathematics

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