Answer :
[tex]\bf \qquad \qquad \textit{inverse proportional variation}\\\\
\textit{\underline{y} varies inversely with \underline{x}}\qquad \qquad y=\cfrac{k}{x}\impliedby
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------\\\\
\begin{cases}
y=20\\
x=3
\end{cases}\implies 20=\cfrac{k}{3}\implies 20\cdot 3=k\implies 60=k
\\\\\\
thus\qquad \boxed{y=\cfrac{60}{x}}\\\\
-------------------------------\\\\
now\qquad x=4\qquad y=\cfrac{60}{4}[/tex]