We have that ratios are proportional if they represent the same relationship. Then, in this case, we have the following expressions:
[tex]\begin{gathered} \frac{3}{4}=\frac{15}{18} \\ \Rightarrow3\cdot18=4\cdot15 \\ \Rightarrow54=60 \end{gathered}[/tex]we have that 54 = 60, which is never true, therefore, 3/4 is not proportional to 15/18
