Mass of the nugget (m) = 100.0 g

Volume of water displaced by the nugget (V) = 15.0 cm³ - 10.0 cm³ = 5.0 cm³

Assuming the nugget is pure gold, we can calculate its density (d) using the formula:

d = m/V

Substituting the values, we get:

d = 100.0 g / 5.0 cm³ = 20.0 g/cm³

The density of pure gold is approximately 19.3 g/cm³.

Therefore, since the calculated density of the nugget (20.0 g/cm³) is greater than the density of pure gold (19.3 g/cm³), we can conclude that the nugget is not pure gold.

It is likely that the nugget is a mixture of gold and other materials, known as fool's gold.