You may have multiplied the 4 pounds by $\displaystyle\frac{{{1}\ \text{ pound}}}{{{16}\ \text{ ounces}}}\usepackage{cmbright}\Huge\$ before adding it to the 7 ounces. You should multiply by $\displaystyle\frac{{{16}\ \text{ ounces}}}{{{1}\ \text{ pound}}}\usepackage{cmbright}\Huge\$ . The correct answer is 71 ounces.