Final answer:
The speed of the train is calculated by dividing the total distance traveled, 390 miles, by the total time taken, 3 hours, resulting in an average speed of 130 mph.
Step-by-step explanation:
To calculate the speed of the train in miles per hour (mph), we can use the formula speed = distance/time. In this case, the train went 390 miles in 3 hours. Using the formula, we take the total distance (390 miles) and divide it by the total time (3 hours).
Speed = 390 miles / 3 hours = 130 mph.
Thus, the correct answer is (a) 130 mph. It's important to always check if the answer is reasonable based on the information given, and in this case, if a train travels 390 miles in 3 hours, it is reasonable to expect that the speed would be 130 mph which is more than 100 miles per hour.