In this case, you must clear the fraction by multiplying them by a common denominator. $\displaystyle{5}{x}\usepackage{cmbright}\Huge\$ is a common factor, not a common denominator, of $\displaystyle{10}{x}\usepackage{cmbright}\Huge\$ and $\displaystyle{25}{x}\usepackage{cmbright}\Huge\$ .