The formula with 20 substituted
US gallons = 20 ÷ 3.785411784 = 5.283
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Litres to Gallons Converter explains where each comes from and gives a longer table.
20 litres is 5.283 US gallons and 4.399 imperial gallons. The imperial gallon is 4.54609 litres against the US gallon's 3.785411784, which is why the two answers differ by a fifth.
What 20 litres means in practice
20 litres is 5.283 US gallons or 4.399 imperial gallons, the capacity of the classic military jerry can and roughly a third to a half of a small car's tank. At 7 L/100km it covers 286 km, about 178 miles.