Final answer:
The average speed of the train is found by dividing the distance traveled by the time taken. The train travels 90 miles in 4/5 of an hour, resulting in an average speed of 112.5 miles per hour.
Step-by-step explanation:
To find the average speed of the train, we divide the total distance traveled by the total time taken. The question states that the train travels 90 miles in 4/5 of an hour. To calculate the average speed, we take the distance (90 miles) and divide it by the time (4/5 hour), giving us:
Speed = Distance / Time
Speed = 90 miles / (4/5 hour)
Speed = 90 miles / 0.8 hour
Speed = 112.5 miles per hour (mph)
Therefore, the correct answer is (b) 112.5 mph, which is the average speed of the train.