P_{n+4}=20P_{n+2}\\ (n+4)!=20*(n+2)!\\ (n+2)!(n+3)(n+4)=20(n+2)!\\ (n+3)(n+4)=20\\ n^2+7n-8=0\\ D=49+4*1*8 = 9^2\\ n=1\\
P_{n+4}=20P_{n+2}\\ (n+4)!=20*(n+2)!\\ (n+2)!(n+3)(n+4)=20(n+2)!\\ (n+3)(n+4)=20\\ n^2+7n-8=0\\ D=49+4*1*8 = 9^2\\ n=1\\