[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}\\\\
-----------------------------\\\\[/tex]
[tex]\bf \textit{l varies directly to V}\implies l=kV
\\\\\\
\textit{now, we also know that }
\begin{cases}
l=4\\
V=100
\end{cases}\implies 4=k100
\\\\\\
\cfrac{4}{100}=k\implies \cfrac{1}{25}=k\impliedby \textit{constant of variation}
\\\\\\
thus\implies l=\cfrac{1}{25}V
\\\\\\
\textit{what is the current when the voltage is 75?}\qquad l=\cfrac{1}{25}\cdot 75[/tex]
