#### Answer

The constant of variation is $180$.
The inverse variation equation is $y=\frac{180}{x}$.

#### Work Step by Step

$y=\frac{k}{x}$
$y=20; x=9$
$20=\frac{k}{(9)}$
$20\times9=k$
$180=k$
The constant of variation is $180$.
The inverse variation equation is $y=\frac{180}{x}$.