The formula with 25 substituted
L/100km = 235.214583 ÷ 25 = 9.41
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 25 mpg (US) is 30.02 mpg on the imperial gallon, 9.41 L/100km and 10.63 km/L. If your 25 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 11.30 L/100km instead.
What 25 mpg (US) means in practice
25 mpg is the round figure that mid-size SUVs and minivans cluster around. At 9.41 L/100km it is 2 litres worse per 100 km than a 30 mpg sedan, which over 15,000 km a year is 300 litres of fuel.