15 is a factor of $\displaystyle{15}{{x}}^{{4}}\usepackage{cmbright}\Huge\$ but not of 20, so it is not a common factor. If you used the distributive property to multiply $\displaystyle{15}{\left({{x}}^{{4}}+{5}\right)}\usepackage{cmbright}\Huge\$ , you would get $\displaystyle{15}{{x}}^{{4}}+{75}\usepackage{cmbright}\Huge\$ , not $\displaystyle{15}{{x}}^{{4}}+{20}\usepackage{cmbright}\Huge\$ .