You may have thought that variation meant the same as difference, and subtracted 9 from 30. An inverse variation equation is $\displaystyle{y}=\frac{{k}}{{x}}\usepackage{cmbright}\Huge\$ or $\displaystyle{k}={x}{y}\usepackage{cmbright}\Huge\$ . Substitute the given values in for x and y, and then solve for k.