A city is planning a project that uses only Granite and Sandstone. The table below shows four rock types that will be considered for use in the project. Also shown for each rock type is its weight per volume, in pounds per cubic foot (lb/ft), and the cost per pound, in dollars.
| Rock type | Weight per volume (lb/ft) | Cost per pound |
|---|---|---|
| Basalt | 180 | 0.09 |
| Limestone | 120 | 0.22 |
The total weight of the rocks for this project is 1,000 pounds. If the volume of Granite used is twice the volume of Sandstone used, which of the following equations could be used to find , the volume in cubic feet, of Sandstone used?
