Add the first two numbers. Because they are both negative, you need to add their absolute values ( $\displaystyle{30}+{15}={45}\usepackage{cmbright}\Huge\$ ) and place a negative sign on the result: $\displaystyle-{45}\usepackage{cmbright}\Huge\$ . Now add $\displaystyle-{45}+{80}\usepackage{cmbright}\Huge\$ . Because the two numbers have different signs, you need to find the difference of their absolute values ( $\displaystyle{80}-{45}={35}\usepackage{cmbright}\Huge\$ ). Since $\displaystyle{\left|{80}\right|}>{\left|-{45}\right|}\usepackage{cmbright}\Huge\$ , the result is positive.