A train leaves Little Rock Arkansas and travels north at 75 kilometers per hour. Another train leaves at the same time and travels south at 85 kilometers per hour. How long will it take before they are 640 kilometers apart?

it will take them 4 hours to be 640 kilometers apart.

Step-by-step 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

b is the correct answer