First find how many cars have valid tickets: 75% * 300 cars = <<75*.01*300=225>>225 cars
Then find the percentage of cars with parking passes: 75% / 5 = 15%
Then multiply that number by the total number of cars to find the number with parking passes: 15% * 300 cars = <<15*.01*300=45>>45 cars
Then subtract all the cars with valid forms of parking to find the number parked illegally: 300 cars - 225 cars - 45 cars = <<300-225-45=30>>30 cars
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