15.31 Pollutants in auto exhausts. In 2017, the entire fleet of light-duty vehicles sold in the United States by each manufacturer must emit an average of no more than 86 milligrams per mile (mg/mi) of nitrogen oxides (NOX) and nonmethane organic gas (NMOG) over the useful life (150,000 miles of driving) of the vehicle. NOX + NMOG emissions over the useful life for one car model vary Normally with mean 80 mg/mi and standard deviation 4 mg/mi. (b) A company has 25 cars of this model in its fleet. What is the probability that the average NOX + NMOG level of these cars is above 86 g/mi?