[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} \\\\[-0.35em] \rule{34em}{0.25pt}[/tex]
[tex]\bf (\stackrel{x}{4},\stackrel{y}{20})\qquad \textit{we know that } \begin{cases} x=4\\ y=20 \end{cases}\implies 20=k4\implies \cfrac{20}{4}=k \\\\\\ 5=k~\hspace{10em} therefore\qquad \boxed{y=5x} \\\\\\ (\stackrel{x}{1},\stackrel{y}{n})~~\textit{when x = 1, what is \underline{y}?}\qquad y=5(1)\implies \stackrel{n}{y}=5[/tex]
