Answer
$k=0.2$
Work Step by Step
Since $y$ varies directly as the square root of $x$, then, $
y=k\sqrt{x}
.$ Substituting the given values of the variables, then the value of $k$ is,
\begin{array}{l}
0.4=k\sqrt{4}
\\\\
0.4=k(2)
\\\\
\dfrac{0.4}{2}=k
\\\\
0.2=k
\\\\
k=0.2
.\end{array}
