Given that y is inversely proportional to x, then they satisfy the following equation:
![y=(k)/(x)](https://img.qammunity.org/2023/formulas/mathematics/college/553kf23di4ua2hg2wlq29izw2itydfmguh.png)
where k is the constant of proportionality.
Substituting with y = 18 and x = 9, we get:
![\begin{gathered} 18=(k)/(9) \\ 18\cdot9=k \\ 162=k \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/3b7fly1ujhtxocrjnb1hzwg4ayekdb2z2e.png)
Therefore, the variation equation in terms of x is:
![y=(162)/(x)](https://img.qammunity.org/2023/formulas/mathematics/college/irb9lm9m8f9nus9a5jaer05us0emv0oho5.png)
Substituting with x = 40, the value of y is:
![\begin{gathered} y=(162)/(40) \\ y=4.05 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/mcc0mbupw5zk6inrvsz7xk5be6gm0i9nzt.png)