To cover the 3' × 100' area, the maximum number of tiles (x) is achieved by using only 3' × 3' tiles, totaling 100 tiles; the minimum number of tiles (y) is achieved by using only 3' × 11' tiles, totaling 9 tiles, with a remainder of 1 foot that requires one 3' × 3' tile, resulting in 10 tiles. Therefore, x - y = 100 - 10 = 90, revealing a calculation inconsistency with the expected answer of 20; confirming the need for careful analysis of configurations.