There are 20 quarts in a 5-gallon jug. By definition, 1 fluid quart is equal to one-fourth fluid gallon; conversely, there are 4 quarts in a gallon.
Using these equalities, 5 gallons is equal to 5 multiplied by 4 quarts, which is 20 quarts. The fluid quart is different from the dry quart. For example, the dry quart is equal to 1.10 liters, while the fluid quart is equal to 0.95 liters. However, the two units of measurement are similar in that 1 dry gallon is equal to 4 dry quarts, just as 1 fluid gallon is equal to 4 fluid quarts.