Let $\displaystyle{c}=\usepackage{cmbright}\Huge\$ the number of cherry pies sold and $\displaystyle{p}=\usepackage{cmbright}\Huge\$ the number of peach pies sold. Then the information leads you to two equations:

$\displaystyle{c}+{p}={25}\usepackage{cmbright}\Huge\$ and $\displaystyle{4}{c}+{3}{p}={84}\usepackage{cmbright}\Huge\$

Multiply the first equation by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ : $\displaystyle-{3}{c}-{3}{p}=-{75}\usepackage{cmbright}\Huge\$

Add this to the second equation: $\displaystyle{c}={9}\usepackage{cmbright}\Huge\$

Substitute into the first equation: $\displaystyle{9}+{p}={25}\usepackage{cmbright}\Huge\$ , so $\displaystyle{p}={16}\usepackage{cmbright}\Huge\$