[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{y} varies jointly as \underline{x} and \underline{z}}\implies y=kxz
\\\\\\
\textit{we also know that }
\begin{cases}
y=80\\
x=5\\
z=4
\end{cases}\implies 80=k(5)(4)\implies 80=20k
\\\\\\
\cfrac{80}{20}=k\implies 4=k\qquad thus\qquad \boxed{y=4xz}\\\\
-------------------------------\\\\ if~\begin{cases}
x=4\\
z=6
\end{cases}~\textit{what is \underline{y}?}\qquad y=4(4)(6)[/tex]
