## Algebra and Trigonometry 10th Edition

$n=10$
$_{n}P_k=\frac{n!}{(n-k)!}$ Hence here $_{n}P_4=\frac{n!}{(n-4)!}=10_{n-1}P_3=10\frac{(n-1)!}{((n-1)-3)!}\\\frac{n!}{(n-4)!}=10\frac{(n-1)!}{(n-4)!}\\n(n-1)(n-2)(n-3)=10(n-1)(n-2)(n-3)\\n=10$