You already have a square yard of topsoil, so how does 20 inches of depth get you the volume of topsoil? There are 36 inches in a yard, and you have 20 inches of topsoil depth that's 20/36ths of a yard of topsoil depth. That's 0.555... or about 0.56 yards. The volume of topsoil is 1 yard by 1 yard by 0.56 yards, or about 0.56 cubic yards of topsoil.
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