[prithibi]

4. answer: n=4
(2n+1)P(n-1): (2n-1)P(n)=3:5
=>(2n+1)!/(2n+1-n+1)!: (2n-1)!(2n-1-n)!=3/5
=>(2n+1)!/(n+2)!: (2n-1)!/(n-1)!=3/5
=>(2n+1)!(n-1)!/(n+2)!(2n-1)!=3/5
=>(2n+1)2n/(n+2)(n+1)n=3/5
=>3n^2-11n-4=0
n= 4 or -1/3
n can not take negative value and n= 4
got it?