12)
[tex]\bf \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{we know that }
\begin{cases}
x=20\\
y=4
\end{cases}\implies 4=20k
\\\\\\
\cfrac{4}{20}=k\implies \cfrac{1}{5}=k\qquad thus\implies y=\cfrac{1}{5}x\\\\
-----------------------------\\\\
\textit{what is "y" when x = 35?}\implies y=\cfrac{1}{5}\cdot 35[/tex]
--------------------------------------------------------------
13)
[tex]\bf \begin{array}{llllll}
\textit{something}&&\textit{varies inversely to}&\textit{something else}\\ \quad \\
\textit{something}&=&\cfrac{{{\textit{some value}}}}{}&\cfrac{}{\textit{something else}}\\ \quad \\
y&=&\cfrac{{{\textit{k}}}}{}&\cfrac{}{x}
\\
&&y=\cfrac{{{ k}}}{x}
\end{array}\\\\
-----------------------------\\\\
[/tex]
[tex]\bf \textit{now, we know again that }
\begin{cases}
x=20\\
y=4
\end{cases}\implies 4=\cfrac{k}{20}
\\\\\\
4\cdot 20=k\implies 80=k\qquad thus\implies y=\cfrac{80}{x}\\\\
-----------------------------\\\\
\textit{what's "y" when x=35?}\qquad y=\cfrac{80}{35}[/tex]
