Question 945597
<pre>
{{{(matrix(2,1,n,2))}}}{{{""-""}}}{{{(matrix(2,1,n,1))}}}{{{""=""}}}{{{20}}}

{{{n!/(2!*(n-2)!)}}}{{{""-""}}}{{{n!/(1!*(n-1)!)}}}{{{""=""}}}{{{20}}}

Replace n! by n*(n-1)*(n-2)! in the first fraction and by n*(n-1)! in the second 

{{{(n*(n-1)*(n-2)!)/(2*(n-2)!)}}}{{{""-""}}}{{{(n*(n-1)!)/(1(n-1)!)}}}{{{""=""}}}{{{20}}}

{{{(n*(n-1)*cross((n-2)!))/(2*cross((n-2)!))}}}{{{""-""}}}{{{(n*cross((n-1)!))/(1cross((n-1)!))}}}{{{""=""}}}{{{20}}}

{{{(n*(n-1))/2}}}{{{""-""}}}{{{n/1}}}{{{""=""}}}{{{20}}}

Multiply through by 2 to clear of fractions:

{{{n*(n-1)-2n}}}{{{""=""}}}{{{40}}}

{{{n^2-n-2n}}}{{{""=""}}}{{{40}}}

{{{n^2-3n}}}{{{""=""}}}{{{40}}}

{{{n^2-3n-40}}}{{{""=""}}}{{{"0"}}}

{{{(n-8)(n+5)}}}{{{""=""}}}{{{"0"}}}

n-8 = 0;  n+5 = 0
  n = 8     n = -5

Ignore negative answer.

Answer:  n = 8

Edwin</pre>