The formula with 28 substituted
L/100km = 235.214583 ÷ 28 = 8.40
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 28 mpg (US) is 33.63 mpg on the imperial gallon, 8.40 L/100km and 11.90 km/L. If your 28 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 10.09 L/100km instead.
What 28 mpg (US) means in practice
28 mpg (US) is a common rating for compact crossovers and a good result for a mid-size sedan with all-wheel drive. It is 8.40 L/100km, or 33.6 mpg in imperial terms.