Final answer:
To find the total distance Mr. Paulson will have traveled after an additional hour, we calculate his average speed and multiply it by the total time. His average speed is 80 miles per hour, so in five hours, he will have traveled 400 miles in total.
Step-by-step explanation:
The question asks us to calculate how far Mr. Paulson will have traveled after an additional hour on the train if he continues at the same speed. To find this, we first determine Mr. Paulson's average speed by dividing the total distance traveled so far (320 miles) by the time taken (4 hours), which equals 80 miles per hour. If he travels for an additional hour at this average speed, he will cover 80 more miles.
Thus, the total distance after five hours will be the sum of the 320 miles already traveled and the 80 miles covered in the fifth hour, giving us a total of 400 miles. Therefore, the answer is (b) 400 miles.