The formula with 50 substituted
L/100km = 235.214583 ÷ 50 = 4.70
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 MPG to L/100km Converter explains where each comes from and gives a longer table.
The same 50 mpg (US) is 60.05 mpg on the imperial gallon, 4.70 L/100km and 21.26 km/L. If your 50 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 5.65 L/100km instead.
What 50 mpg (US) means in practice
50 mpg (US) is the psychological line for hybrids and it converts to 4.70 L/100km. It is 60 mpg in imperial gallons, and a car that returns it costs roughly half as much to fuel as the American average.