Let $\displaystyle{S}=\usepackage{cmbright}\Huge\$ the cost of one ounce of Sumatra and $\displaystyle{C}=\usepackage{cmbright}\Huge\$ the cost of one ounce of Celebes Kalossi. Then the information leads you to two equations:

$\displaystyle{12}{S}+{4}{C}={32}\usepackage{cmbright}\Huge\$ and $\displaystyle{4}{S}+{12}{C}={40}\usepackage{cmbright}\Huge\$

Multiply the first equation by $\displaystyle-{3}\usepackage{cmbright}\Huge\$ : $\displaystyle-{36}{S}-{12}{C}=-{96}\usepackage{cmbright}\Huge\$

Add this to the second equation: $\displaystyle-{32}{S}=-{56}\usepackage{cmbright}\Huge\$ , so $\displaystyle{S}={1.75}\usepackage{cmbright}\Huge\$

Substitute into the first equation: $\displaystyle{12}{\left({1.75}\right)}+{4}{C}={32}\usepackage{cmbright}\Huge\$ or $\displaystyle{21}+{4}{C}={32}\usepackage{cmbright}\Huge\$

This gives you $\displaystyle{4}{C}={11}\usepackage{cmbright}\Huge\$ , so $\displaystyle{C}={2.75}\usepackage{cmbright}\Huge\$ .