Use a joint variation equation, $\displaystyle{A}={k}{x}{z}\usepackage{cmbright}\Huge\$ :

$\displaystyle{150}={k}{\left({20}\right)}{\left({15}\right)}\usepackage{cmbright}\Huge\$ , so $\displaystyle{k}=\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$

Substitute k in a new joint variation equation using $\displaystyle{x}={30}\usepackage{cmbright}\Huge\$ and $\displaystyle{z}={18}\usepackage{cmbright}\Huge\$ :

$\displaystyle{A}=\frac{{1}}{{2}}{\left({30}\right)}{\left({18}\right)}\usepackage{cmbright}\Huge\$ , so $\displaystyle{A}={270}\usepackage{cmbright}\Huge\$