Answer
$z=0.5xy$
Work Step by Step
$z$ is directly proportional to the product of $x$ and $y$.
This means that the variables are related by the equation $z=kxy$ where $k$ is the constant of proportionality.
When x=4 and y=5, z=10. Substitute these values into the equation above to have:
$z=kxy
\\10=k(4)(5)
\\10=20k
\\\frac{10}{20}=k
\\0.5=k$
The constant of proportionality is $0.5$
Therefore, the three variables are related by the equation
$z=0.5xy$