The number of grey tiles can be found thusly :
Let n be the number of rows of tiles
The number of yellow tiles = n + (n -1) = 81
So
2n - 1 = 81
2n = 82
n = 41 rows
Number of grey tiles = ( n - 2) ( n - 1) + (n - 1) = ( n - 1) ( n - 2 + 1) = ( n - 1) ( n -1) =
(n -1)^2 = (41 -1) ^2 = 40^2 = 1600