The formula with 57 substituted
L/100km = 235.214583 ÷ 57 = 4.13
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 57 mpg (US) is 68.45 mpg on the imperial gallon, 4.13 L/100km and 24.23 km/L. If your 57 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 4.96 L/100km instead.
What 57 mpg (US) means in practice
57 mpg (US), or 4.13 L/100km, is the combined rating of the current Toyota Prius in its most efficient form, and the highest figure a petrol car without a plug achieves on the EPA cycle. In imperial terms it is 68.5 mpg.