You may have made a mistake when multiplying the numerator by $\displaystyle\sqrt{{{5}}}\usepackage{cmbright}\Huge\$ , obtaining $\displaystyle\frac{{\sqrt{{{20}}}-\sqrt{{{15}}}}}{\sqrt{{{25}}}}\usepackage{cmbright}\Huge\$ and then simplifying. You should have obtained $\displaystyle\frac{{{4}\sqrt{{{5}}}-\sqrt{{{15}}}}}{\sqrt{{{25}}}}\usepackage{cmbright}\Huge\$ and then simplified.