16m？ = =4m×4m, and the side length of a small square is 4m.

﹙5-4﹚×5÷2+4×4÷2=2.5+8= 10.5m？ , the sum of the areas of the two blank triangles above.

﹙5+4﹚×4÷2=9×4÷2= 18m？ , the area of the big blank triangle below.

25+ 16- 10.5- 18 = 12.5m？ The shadow area is 12.5 square meters.