I might have no idea what I'm saying but here goes. n = 5. Since the 3 X 3 grid is chosen from the 5 x 5 grid or 3 x 3 grid is chosen from the n x n grid. If g = 11, and x = 1, then x+2n or 1+2(5) = 11. And if i = 13. Then x+2+2n or 1+2+2(5) = 13. Since cg-xi = 20, and n = 5, then 4n or 4(5) equals 20.

I hope that helped?