He needs 20 pints.

$\displaystyle\frac{{{3}\ \text{ gallons}}}{{1}}\cdot\frac{{{4}\ \text{ quarts}}}{{{1}\ \text{ gallon}}}\cdot\frac{{{2}\ \text{ pints}}}{{{1}\ \text{ quart}}}={24}\ \text{ pints}\usepackage{cmbright}\Huge\$ , while

$\displaystyle\frac{{{2}\ \text{ gallons}}}{{1}}\cdot\frac{{{4}\ \text{ quarts}}}{{{1}\ \text{ gallon}}}\cdot\frac{{{2}\ \text{ pints}}}{{{1}\ \text{ quart}}}={16}\ \text{ pints}\usepackage{cmbright}\Huge\$