Answer
$k=\frac{1}{5}$, $y=\frac{1}{5}x$
Work Step by Step
If y varies directly as x, we know that the variables x and y can be related by the function $y=kx$, where k is the constant of proportionality.
If $y=4$ when $x=20$, then we know that $4=20k$. Divide both sides by 20.
$k=\frac{4}{20}=\frac{1}{5}$
Therefore, the constant of variation is $k=\frac{1}{5}$ and the direct variation equation is $y=\frac{1}{5}x$.