Answer:
y=2xz
Explanation:
Given that y varies jointly with x and z, the equation of variation is given as:
[tex]\begin{gathered} y=kxz \\ \text{where k is the constant of variation} \end{gathered}[/tex]When x=5, z=9, and y=90:
[tex]\begin{gathered} y=kxz \\ 90=k\times5\times9 \\ 90=45k \\ \text{Divide both sides by 45} \\ \frac{90}{45}=\frac{45k}{45} \\ k=2 \end{gathered}[/tex]Therefore, an equation describing the relationship of the given variables is:
[tex]y=2xz[/tex]