The formula with 24 substituted
L/100km = 235.214583 ÷ 24 = 9.80
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 24 mpg (US) is 28.82 mpg on the imperial gallon, 9.80 L/100km and 10.20 km/L. If your 24 mpg figure came from a British source it already uses the imperial gallon, and the metric answer is 11.77 L/100km instead.
What 24 mpg (US) means in practice
24 mpg (US) is 9.80 L/100km, the combined rating of many turbocharged three-row SUVs and a fair number of sports cars. It is about where the American new-vehicle average stood in the early 2010s.