[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 also know that }
\begin{cases}
y=15\\
x=6
\end{cases}\implies 15=k6\implies \cfrac{15}{6}=k
\\\\\\
therefore\qquad \boxed{y=\cfrac{15}{6}x}\\\\
----------------------------\\\\
\textit{what's \underline{y} when x=7?}\qquad y=\cfrac{15}{6}\cdot 7[/tex]
