Question 1036831
<pre><b>
For all positive integers n and r, where n &#8805; r &#8805; 0
nCk = nC(n-k)

If you mean 20Cr = 20C(r+2), then since r+2 > r, we turn nCk = nC(n-k)
around as
  
 nC(n-k) =  nCk
20Cr     = 20C(r+2)

n=20, r=n-k, k=r+2
      r=20-k
      r=20-(r+2)
      r=20-r-2
      r=18-r
     2r=18
      r=9

Checking
  20Cr = 20C(r+2)
  20C9 = 20C(9+2)
  20C9 = 20C11
167960 = 167960

Edwin</pre>