You may have multiplied 78 inches by $\displaystyle\frac{{{12}\ \text{ inches}}}{{{1}\ \text{ foot}}}\usepackage{cmbright}\Huge\$ . You need to multiply 78 inches by $\displaystyle\frac{{{1}\ \text{ foot}}}{{{12}\ \text{ inches}}}\usepackage{cmbright}\Huge\$ to get the correct answer.