Answer
$5\text{ gallons}$
Work Step by Step
Use the conversion rate $1 \text{gallon} = 4\text{ quarts}$ by multiplying $\frac{1\text{ gallon}}{4\text{ quarts}}$ to obtain:
$\require{cancel}
20\text{ quarts}
\\= 20\text{ quarts} \cdot \frac{1\text{ gallon}}{4\text{ quarts}}
\\= \cancel{20\text{ quarts}}^5 \cdot \frac{1\text{ gallon}}{\cancel{4\text{ quarts}}}
\\=5\text{ gallons}$
