The minimum speed to be maintained by a vehicle traveling on the expressway is 150 miles per hour. The question does not specify the maximum speed limit on the expressway. The car has to travel 320 miles on the expressway. To maintain a minimum speed of 150 mph, the maximum time that can be taken by the car is 320/150 = 2.13 hours.
The maximum time that the car can travel for to maintain a speed above 150 mph is 2.13 hours.
The minimum time taken by the car cannot be determined as the information provided is insufficient.