# Thread: Need help finding "n" in Permutations!

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

2. Originally Posted by krzyrice

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

nP8=13 $\bullet$ nP7