[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 \textit{\underline{z} is directly proportional to \underline{x}}\qquad z=kx
\\\\\\
\textit{we also know that }
\begin{cases}
x=12\\
z=228
\end{cases}\implies 228=k12\implies \cfrac{228}{12}=k
\\\\\\
19=k\qquad thus\qquad \boxed{z=19x}\\\\
-------------------------------\\\\
\textit{what's \underline{z} when \underline{x} is 18?}\qquad z=19(18)[/tex]
