since we know is a proportionality, namely a multiplier on "x", we can simply use any of those points, say hmmm let's use 10, 15,
[tex]\bf \qquad \qquad \textit{direct proportional variation}\\\\
\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------\\\\
\textit{we know that }
\begin{cases}
x=10\\
y=15
\end{cases}\implies 15=k10\implies \cfrac{15}{10}=k\implies \cfrac{3}{2}=k[/tex]
