[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}\\\\
------------------------------- [/tex]
[tex] \bf \stackrel{\textit{\underline{m}illigrams of a certain medicine varies directly with the \underline{w}eight}}{m=kw}
\\\\\\
\textit{we also know that }
\begin{cases}
w=30\\
m=\frac{3}{5}
\end{cases}\implies \cfrac{3}{5}=k30\implies \cfrac{3}{5\cdot 30}=k
\\\\\\
\cfrac{1}{50}=k\qquad therefore\qquad \boxed{m=\cfrac{1}{50}w} [/tex]
