Step 1 :Given the total number of vehicles is 2504, the total number of trucks is 991, the number of trucks that did not use a toll pass is 648, and the total number of vehicles that did not use a toll pass is 1608.
Step 2 :The probability that a randomly selected vehicle is a truck can be calculated by dividing the total number of trucks by the total number of vehicles. So, the probability is \(\frac{991}{2504} = 0.3957667731629393\).
Step 3 :The conditional probability that a vehicle is a truck given that it did not use a toll pass can be calculated by dividing the number of trucks that did not use a toll pass by the total number of vehicles that did not use a toll pass. So, the probability is \(\frac{648}{1608} = 0.40298507462686567\).
Step 4 :Rounding to four decimal places, the probability that the vehicle was a truck is \(\boxed{0.3958}\).
Step 5 :The probability that the vehicle was a truck, given that the vehicle did not use a toll pass, is \(\boxed{0.4030}\).