The formula with 15 substituted
L/100km = 235.214583 ÷ 15 = 15.68
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 15 mpg (US) is 18.01 mpg on the imperial gallon, 15.68 L/100km and 6.38 km/L. If your 15 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 18.83 L/100km instead.
What 15 mpg (US) means in practice
15 mpg (US) is what large body-on-frame SUVs and V8 pickups returned a generation ago, and what a few still return when towing. At 15.68 L/100km it uses twice the fuel of a 30 mpg car for the same trip.