Thread: Solve for n, permutation problem

Solve for n.
P(n,5)=20P(n,3)

I'm leaning this course by myself, and i'm finding it difficult to understand how to do this problem with the material that was given to me in my book. Any help would be greatly appreciated because so far all i'm understanding is that i plug the numbers in to n!/(n-r)!

Originally Posted by youvy

Solve for n.
P(n,5)=20P(n,3)

I'm leaning this course by myself, and i'm finding it difficult to understand how to do this problem with the material that was given to me in my book. Any help would be greatly appreciated because so far all i'm understanding is that i plug the numbers in to n!/(n-r)!

n!/(n-5)! = 20n!/(n-3)!

1/(n-5)! = 20/(n-3)!

1 = 20/ (n-4)(n-3)

n^2 - 7n + 12 = 20

n^2 - 8n + n - 8 =0

(n -8)(n+1) =0

Thus answer is 8