Opposites have the same absolute value and different signs. $\displaystyle{\left|{15}\right|}\usepackage{cmbright}\Huge\$ and $\displaystyle{\left|-{15}\right|}\usepackage{cmbright}\Huge\$ are both equal to $\displaystyle{15}\usepackage{cmbright}\Huge\$ . The numbers $\displaystyle{15}\usepackage{cmbright}\Huge\$ and $\displaystyle-{15}\usepackage{cmbright}\Huge\$ also have different signs.