The situation can be represented by the following equation:
[tex]z=\frac{k}{xy}[/tex]
where k is a constant of proportionality.
Plugging in the given values:
[tex]0.5=\frac{k}{2\times4}[/tex]
from which k = 4.
When x = 4 and y = 9
[tex]z=\frac{4}{4\times9}=\frac{1}{9}[/tex]
