Do not forget to change the numerators when rewriting fractions to have common denominators. Change $\displaystyle\frac{{2}}{{5}}\usepackage{cmbright}\Huge\$ to $\displaystyle\frac{{8}}{{20}}\usepackage{cmbright}\Huge\$ and $\displaystyle\frac{{1}}{{4}}\usepackage{cmbright}\Huge\$ to $\displaystyle\frac{{5}}{{20}}\usepackage{cmbright}\Huge\$ before subtracting.