[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}\\\\
\rule{31em}{0.25pt}\\\\
\textit{we also know that }
\begin{cases}
y=25\\
x=5
\end{cases}\implies 25=k5\implies \cfrac{25}{5}=k
\\[2em]
5=k\hspace{20em}\boxed{y=5x}
\\[2em]
\textit{when x = 25, what is \underline{y}?}\qquad y=5(25)\implies y=125 [/tex]
