You may have set up the proportion incorrectly. $\displaystyle{\overline{{{B}{L}}}}\usepackage{cmbright}\Huge\$ corresponds to $\displaystyle{\overline{{{A}{H}}}}\usepackage{cmbright}\Huge\$ , and $\displaystyle{\overline{{{B}{T}}}}\usepackage{cmbright}\Huge\$ corresponds to $\displaystyle{\overline{{{A}{M}}}}\usepackage{cmbright}\Huge\$ . Now set up the correct proportion, cross-multiply, and solve.