[tex]\bf \qquad \qquad \textit{double proportional variation}
\\\\
\begin{array}{llll}
\textit{\underline{y} varies directly with \underline{x}}\\
\textit{and inversely with \underline{z}}
\end{array}\implies y=\cfrac{kx}{z}\impliedby
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------[/tex]
[tex]\bf \textit{\underline{y} varies jointly with \underline{w} and \underline{x} inversely with \underline{z}}\qquad y=\cfrac{kwx}{z}
\\\\\\
\textit{we also know that }
\begin{cases}
y=360\\
w=6\\
x=20\\
z=3
\end{cases}\implies 360=\cfrac{k(6)(20)}{3}
\\\\\\
\cfrac{360(3)}{(6)(20)}=k\implies 9=k\qquad therefore\qquad \boxed{y=\cfrac{9wx}{z}}[/tex]
