[tex]\bf \qquad \textit{direct proportional variation}\\\\
\begin{array}{cccccclllll}
\textit{something}&&\textit{varies directly to}&&\textit{something else}\\ \quad \\
\textit{something}&=&{{ \textit{some value}}}&\cdot &\textit{something else}\\ \quad \\
y&=&{{ k}}&\cdot&x
\\
&& y={{ k }}x
\end{array}\\\\
-----------------------------\\\\[/tex]
[tex]\bf \textit{x varies directly with y and z}\implies x=\underline{k}yz\qquad \underline{k}=
\begin{array}{llll}
constant\ of\\
variation
\end{array}
\\\\\\
\textit{we also know that }
\begin{cases}
x=120\\
y=5\\
z=8
\end{cases}\implies 120=k(5)(8)\implies \cfrac{120}{5\cdot 8}=k
\\\\\\
\boxed{3=k}\qquad thus\implies x=\underline{3}yz
\\\\\\
\textit{now what's "x" when }
\begin{cases}
y=6\\
z=2
\end{cases}\implies x=3(6)(2)[/tex]
