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 90 milligrams per mile (mg/mi) of nitrogen oxides (NOX) and...

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 90 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 of 84 mg/mi and standard deviation 6 mg/mi.

A company has 16 cars of this model in its fleet. What is the probability that the average NOX + NMOG level x¯ of these cars is above 90 mg/mi?