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Permutation and Combinations — Maths STD 11 Science — Question

CBSE BoardEnglish MediumSTD 11 ScienceMathsPermutation and Combinations1 Mark
Question
If $\ ^nP_r= 840, \ ^nC_r = 35,$ then $r =......$
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Answer

If $\ ^nP_r= 840, ^nC_r = 35, $then $r = 4$
We have$, \ ^nP_r = 840$ and $^nC_r = 35$
Now $\ ^nP_r = \ ^nC_r . r!$
$\Rightarrow840=35 \times\text{r!}\ \Rightarrow\ ^.\text{r}!=24$
$\therefore\text{r}=4$

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