Permutations And Combinations Question 242
Question: If $ {}^{n}P_5=20.\ {}^{n}P_3 $ , then $ n= $
Options:
A) 4
B) 8
C) 6
D) 7
Correct Answer: B $ \Rightarrow $ $ (n-3)(n-4)=20\Rightarrow n=-1,\ 8 $ But $ -1 $ is not exceptable.Show Answer
Answer:
Solution:
$ \frac{n\ !}{(n-5)\ !}\times \frac{(n-3)\ !}{n\ !} $ =20