1/2 cubic foot is 864 cubic inches.
12 in = 1 ft volume = area × depth = 16 sq ft × 2 in = 16 sq ft × (2 ÷ 12) ft = 2⅔ cu ft ≈ 2.67 cu ft
2 ft * 13ft * 5 in = 2 ft * 13 ft * 5/12 ft =2*13*5/12 cubic ft = 10.833... cu ft
8ft = 96 inches 96 inches X 96 inches X 2 inches = 18,432 cubic inches 1 cubic foot or 12 inches x 12 inches x 12 inches = 1,728 cubic inches 18,432 cu in / 1,728 cu in = 10.67 cu ft
12 in = 1 ft Volume = 311 sq ft × 2 in = 311 sq ft × (2 ÷ 12) ft = 51 5/6 cu ft (≈ 51.8 cu ft) = 51 5/6 ÷ 0.8 cu ft/bag = 64 19/24 bags ≈ 64.8 bags → you need to buy 65 bags.
2 × 1 cu ft = 2 cu ft.
1 yard = 27 cu ft; 2 yard = 54 cu ft at 2 cu ft per bag, that is 54/2 = 27 bags
I presume the 6 mean 6 inches, so 1/2 ft. The volume of the garden is 4.5 cu ft. So you will need between 2 and 3 bags.
100 cu ft × 2 = 200 cu ft.
12.3 bags for every 2 inches deep. (rounded)
(10x16)/3 =53.3 cu ft 53.3/27=2 cu yards
16553 sq ft * 2 inches = 16553/9 sq yards * 2/36 yards = 16553*2/(9*36) cubic yards = 102.18 cu yards.