# Math Help - Need help finding "n" in Permutations!

1. ## Need help finding "n" in Permutations!

Link 1: http://i42.tinypic.com/zu2fwj.png
Link 2: http://i40.tinypic.com/e8rhvs.png

Please help me. I do not know how to solve for n. Thank you.

2. Originally Posted by krzyrice
Link 1: http://i42.tinypic.com/zu2fwj.png
Link 2: http://i40.tinypic.com/e8rhvs.png

Please help me. I do not know how to solve for n. Thank you.
Link1: $^nP_8 = 13 \cdot ^nP_7$.

Substitute the definition $^nP_r$ and simplify. Then solve the resulting simple linear equation for n. Note that (n - 7)! = (n - 7)(n - 8)!

Link2: $^nP_6 = 90 \cdot ^nP_4$.

Done in a similar way. Yiou will end up with a quadratic equation to solve. Note that one of the solutions to this quadratic equation is not be valid.

3. Originally Posted by krzyrice
Link 1: http://i42.tinypic.com/zu2fwj.png
Link 2: http://i40.tinypic.com/e8rhvs.png

Please help me. I do not know how to solve for n. Thank you.
Link1:

nP8=13 $\bullet$ nP7
n(n-1)(n-2)(n-3)(n-4)(n-5)(n-6)(n-7)=13n(n-1)(n-2)(n-3)(n-4)(n-5)(n-6)
n-7=13
n=

Link2:
Use the same concept but remember n should be greater than 6.

4. thank you so much for the help guys