Here, we want to solve a proportional relationship problem
a) We have the proportion as follows;
[tex]\begin{gathered} x\propto\frac{1}{y} \\ \\ k\text{ as constant} \\ \\ k\text{ = xy} \\ \\ \text{for x = 18 and y = 4} \\ \\ k\text{ = 18}\times4\text{ = 72} \\ xy\text{ = 72} \end{gathered}[/tex]b) X, when y = 30
[tex]\begin{gathered} xy\text{ = 72} \\ x\text{ = }\frac{72}{y} \\ x\text{ = }\frac{72}{30} \\ x\text{ = 2.4} \end{gathered}[/tex]