The formula with 50 substituted
US gallons = 50 ÷ 3.785411784 = 13.209
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.
50 litres is 13.209 US gallons and 10.998 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 50 litres means in practice
50 litres is 13.209 US gallons or 10.998 imperial gallons, near enough 11, and it is the fuel tank size of a great many European hatchbacks and compact SUVs. At 6.5 L/100km it gives a range of about 770 km, or 478 miles.