Linear equations in two variables SAT question

Choice C is correct. It’s given that the weight of the granite used in the garden is 330 pounds. The weight of the limestone used in the garden is a product of its weight per volume, in lb/ft³, and its volume, in ft³. Therefore, the weight of the limestone used in the garden can be represented by , where x is the volume, in ft³, of the limestone used. Similarly, the weight of the basalt used in the garden can be represented by , where y is the volume, in ft³, of the basalt used. It’s given that the total weight of the rocks used in the garden will be 1,000 pounds. Thus, the sum of the weights of the three rock types used is 1,000 pounds, which can be represented by the equation . Subtracting 330 from both sides of this equation yields .

Choice A is incorrect. This equation uses the weight per volume of granite instead of limestone. Choice B is incorrect. This equation uses the weight per volume of granite instead of basalt, and doesn’t take into account the 330 pounds of granite that will be used in the garden. Choice D is incorrect. This equation doesn’t take into account the 330 pounds of granite that will be used in the garden.