Correct. The information gives you two equations:

(A) $\displaystyle{y}={x}+{10}\usepackage{cmbright}\Huge\$ (B) $\displaystyle\frac{{{x}+{y}}}{{2}}={75}\usepackage{cmbright}\Huge\$

Substitute $\displaystyle{x}+{10}\usepackage{cmbright}\Huge\$ for y in (B): $\displaystyle\frac{{{x}+{x}+{10}}}{{2}}={75}\usepackage{cmbright}\Huge\$

Simplify and multiply both sides by 2: $\displaystyle{2}{x}+{10}={150}\usepackage{cmbright}\Huge\$

Subtract 10 and divide by 2: $\displaystyle{x}={70}\usepackage{cmbright}\Huge\$ , so $\displaystyle{y}={70}+{10}={80}\usepackage{cmbright}\Huge\$