You may have correctly simplified the numerator by adding the two fractions, and simplified the denominator by adding the two fractions. However, it appears that instead of dividing $\displaystyle\frac{{4}}{{10}}\usepackage{cmbright}\Huge\$ by $\displaystyle\frac{{3}}{{5}}\usepackage{cmbright}\Huge\$ , you multiplied these two fractions.