$\displaystyle\frac{{{40}\ \text{ cups}}}{{1}}\cdot\frac{{{1}\ \text{ gallon}}}{{{16}\ \text{ cups}}}={2.5}\ \text{ gallons}\usepackage{cmbright}\Huge\$

The amount left over is $\displaystyle{6}-{2.5}={3.5}\ \text{ gallons}\usepackage{cmbright}\Huge\$ .