Answer:
d = D - 80*t
where d is the train's distance to Greensburg, D is the distance from Springfield to Greensburg and t is the time travelled in hours.
Explanation:
Let's call the distance from Springfield to Greensburg D, and the train's distance to Greensburg d.
We know that the train is traveling towards Greensburg, so its distance to Greensburg will decrease over time, so for our equation we must use a negative speed (-80 miles per hour)
We can then write the following equation:
d = D - 80*t
where t is the amount of time the train travelled in hours.