The formula with 42 substituted
L/100km = 235.214583 ÷ 42 = 5.60
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 42 mpg (US) is 50.44 mpg on the imperial gallon, 5.60 L/100km and 17.86 km/L. If your 42 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 6.73 L/100km instead.
What 42 mpg (US) means in practice
42 mpg is 5.60 L/100km, where several mild hybrids and the most efficient small petrol cars land. It is a little below the level at which a hybrid premium pays for itself in fuel.