Since 12 is not a common factor of 18 and 30, $\displaystyle{12}{a}{b}{{c}}^{{3}}\usepackage{cmbright}\Huge\$ is not a common factor of $\displaystyle{18}{{a}}^{{2}}{b}{{c}}^{{4}}\usepackage{cmbright}\Huge\$ and $\displaystyle{30}{a}{{b}}^{{2}}{{c}}^{{3}}\usepackage{cmbright}\Huge\$ .