Basic College Mathematics (10th Edition)

$\frac{3}{4}$
18 carat gold. It has all 18 part gold and 6 parts alloy. $\frac{Parts of Gold}{Parts of gold + parts of alloy} = \frac{18}{18+6} = \frac{18}{24} = \frac{18\div6}{24\div6} = \frac{3}{4}$