If y varies inversely with square x, so
[tex]\begin{gathered} y=\frac{k}{x^2} \\ OR \\ \frac{y_1}{y_2}=\frac{x^2_2}{x^2_1} \end{gathered}[/tex]Where k is the constant of variation, you can get it by using the initial values of x and y
We will use the second rule
Since y is 3 when x is 18 (initial values), so
[tex]\begin{gathered} y_1=3 \\ x_1=18 \end{gathered}[/tex]We need to find y when x is 20
[tex]\begin{gathered} y_2=? \\ x_2=20 \end{gathered}[/tex]Let us substitute them in the second rule
[tex]\begin{gathered} \frac{3}{y_2}=\frac{(20)^2}{(18)^2} \\ \frac{3}{y_2}=\frac{400}{324} \end{gathered}[/tex]By using cross multiplication
[tex]\begin{gathered} 400\times y_2=3\times324 \\ 400y_2=972 \end{gathered}[/tex]Divide both sides by 400
[tex]\begin{gathered} \frac{400y_2}{400}=\frac{972}{400} \\ y_2=2.43 \end{gathered}[/tex]The value of y is 2.43 (There is no necessary to round it)
In direct proportion, if y increasing x also increasing (both increasing or decreasing)
In inverse proportion, if y increasing, x decreasing and vice versa
