[tex]\bf \begin{array}{cccccclllll}
\textit{something}&&\textit{varies directly to}&&\textit{something else}\\ \quad \\
\textit{something}&=&{{ \textit{some value}}}&\cdot &\textit{something else}\\ \quad \\
y&=&{{ k}}&\cdot&x
\\
&& y={{ k }}x
\end{array}\\\\
-----------------------------\\\\
\textit{r varies directly as s}\implies r=ks\qquad k=\textit{constant of variation}
\\\\\\
r=ks\qquad
\begin{cases}
s=13\\
k=15
\end{cases}\implies r=15\cdot 13[/tex]
