The least common multiple of 4 and 16 is 16, so rewrite the fraction $\displaystyle\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ with a denominator of 16. $\displaystyle\frac{{3}}{{4}}\cdot\frac{{4}}{{4}}=\frac{{12}}{{16}}\usepackage{cmbright}\Huge\$