Helena uses a mixture of compost and topsoil in her garden. She purchased a total of 10 cubic yards of compost and topsoil for $180. If compost costs $25 per cubic yard and topsoil costs $15 per cubic yard, how many cubic yards of topsoil did she purchase?

Let c be the cubic yards of compost and t the cubic yards of topsoil. The total amount gives c + t = 10, and the cost gives 25c + 15t = 180. Substituting c = 10 − t into the cost equation gives 250 − 25t + 15t = 180, so t = 7. Helena purchased 7 cubic yards of topsoil, which is choice D.

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