The density and strength of concrete are determined by the ratio of cement and aggregate

  

The density and strength of concrete are determined by the ratio of cement and aggregate (aggregate is sand, gravel, or crushed stone). Suppose that a contractor has 460 ft^3 of a dry concrete mixture that is 70% sand by volume. How much pure sand must be added to form a new mixture that is 80% sand by volume?

The contractor needs ---- of pure sand to form the resulting mixture.