Answer: it will take them 4 hours to be 640 kilometers apart.
Explanation:
Let t represent the time it will take for both trains to be 640 kilometers apart. It means that the total distance that both trains will covered in t hours is 640 kilometers.
Distance = speed × time
A train leaves Little Rock Arkansas and travels north at 75 kilometers per hour. Distance covered after t hours would be 75 × t = 75t
Another train leaves at the same time and travels south at 85 kilometers per hour. Distance covered after t hours would be 85 × t = 85t
Since the total distance covered is 640 kilometers, then
75t + 85t = 640
160t = 649
t = 640/160
t = 4 hours