[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{weight of air is directly proportional to its volume}\qquad W=kV
\\\\\\
\textit{we also know that }
\begin{cases}
V=10\\
W=13
\end{cases}\implies 13=k10\implies \cfrac{13}{10}=k[/tex]
