The information gives you two equations:

$\displaystyle{B}+{R}={120}\usepackage{cmbright}\Huge\$ and $\displaystyle{B}-{R}={10}\usepackage{cmbright}\Huge\$

Add the equations: $\displaystyle{2}{B}={130}\usepackage{cmbright}\Huge\$ , so $\displaystyle{B}={65}\usepackage{cmbright}\Huge\$

Substitute 65 for B in the first equation: $\displaystyle{65}+{R}={120}\usepackage{cmbright}\Huge\$ , so $\displaystyle{R}={55}\usepackage{cmbright}\Huge\$