You may have multiplied the $\displaystyle{3}\frac{{1}}{{2}}\usepackage{cmbright}\Huge\$ pounds by $\displaystyle\frac{{{1}\ \text{ pound}}}{{{16}\ \text{ ounces}}}\usepackage{cmbright}\Huge\$ before adding the pounds to the 4 ounces. You should multiply by $\displaystyle\frac{{{16}\ \text{ ounces}}}{{{1}\ \text{ pound}}}\usepackage{cmbright}\Huge\$ to get the correct number of ounces in any number of pounds.