[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 directly as \underline{x} and \underline{z}}\qquad y=kxz
\\\\\\
\textit{we also know that }
\begin{cases}
y=40\\
x=5\\
z=4
\end{cases}\implies 40=k(5)(4)\implies 40=20k
\\\\\\
\cfrac{40}{20}=k\implies 2=k\qquad therefore\qquad \boxed{y=2xz}
\\\\\\
\textit{now when x = 2 and z = 1, what is \underline{y}?}\qquad y=2(2)(1)[/tex]
