Length = 10 1/8 ft
Width = 7 7/8 ft
Height = 1/3 ft
Vol = Length * Width * Height
V = (81/8)(63/8)(1/3) = (81*63*1)/(8*8*3) = 5103/192
V = 26 37/64 ft^3
So in order to cover the garden with 26 37/64 ft^3 you will need 27 ft^3 of filling.
Answer: 27 ft^3 of filling