This is the fraction of water that has been used. Since 4 ounces have been used, $\displaystyle{12}-{4}={8}\usepackage{cmbright}\Huge\$ ounces remain. This is out of a total of 12 ounces, so the correct fraction is 8 out of 12, or $\displaystyle\frac{{8}}{{12}}\usepackage{cmbright}\Huge\$ .