7.96 The number of vehicles leaving a turnpike at a certain exit during a particular time period has approximately a normal distribution with mean value 500 and standard deviation 75. What is the approximate probability that the number of cars exiting during this period is

a. at least 650?

b. strictly between 400 and 550? (Strictly means that the values 400 and 550 are not included.)

c. between 400 and 550 (inclusive)?