Question
If ${}^n{P_4}\;:\;{}^n{P_5} = 1:2$, then $n = $
✓
Answer
c
(c) $\frac{{n\;!}}{{(n - 4)\;!}} \times \frac{{(n - 5)\;!}}{{n\;!}} = \frac{1}{2}$
(c) $\frac{{n\;!}}{{(n - 4)\;!}} \times \frac{{(n - 5)\;!}}{{n\;!}} = \frac{1}{2}$
$\Rightarrow n - 4 = 2 $
$\Rightarrow n = 6$.
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