y = 4
Explanation:[tex]\begin{gathered} 3\text{ : y = 18 : 24} \\ 3\colon y\text{ = }\frac{3}{y} \\ 18\colon24\text{ = }\frac{18}{24} \end{gathered}[/tex][tex]\begin{gathered} \frac{3}{y}\text{ = }\frac{18}{24} \\ \text{cross mult}iply\colon \\ 3(24)\text{ = y(18)} \\ \text{divide both sides by 18:} \\ \frac{3(24)}{18}\text{ = }\frac{18y}{18} \end{gathered}[/tex][tex]\begin{gathered} y\text{ = }\frac{24}{6} \\ y\text{ = 4} \end{gathered}[/tex]