For variables to be related proportionally, the relationship must have a constant of proportionality. In our case we will represent the constant of proportionality as k. Therefore,
[tex]\begin{gathered} y=kx \\ \text{where} \\ k=\text{constant of proportionality} \\ 10=4k \\ k=\frac{10}{4} \\ k=\frac{5}{2} \end{gathered}[/tex]Now lets find y when x = 18
[tex]\begin{gathered} y=kx \\ y=\frac{5}{2}\times18 \\ y=\frac{90}{2} \\ y=45 \end{gathered}[/tex]