10 ft^2 * 10 ft^2 * 0.4 = infinite cubic yards
10 ft^2 * 10 ft^2 would be 4 dimensions.
Cubic yards are a measure of 3 dimensions.
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10 ft * 10 ft * 0.4 = 40 ft^2
There are 0 cubic yards in 40 ft^2 because 40 ft^2 is a measure of 2 dimensions and cubic yards are a measure of 3 dimensions.
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To cover a garden of 100 ft^2 to 4 inches:
100 ft^2 * 4 inches
100 ft^2 * (1/3) ft
100/3 ft^3
(100/3) / 27 yd^3
100/81 yd^3
about 1.25 yd^3
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To cover a garden of 100 yd^2 to a depth of 4 in:
100 yd^2 * 4in
100 yd^2 * (4/12) ft
100 yd^2 * (4/36) yd
100 yd^2 * (1/9) yd
100/9 yd^3
11 1/9 yd^3