Hi Rachelle
The speed of one train is X and the speed of the other train is X+20 = each train's rate
T = time = 3 hours
D1 + D2 = 600 miles where D1 and D2 are the distances the two trains travel in the 3 hours
Since D = RT
3X + 3(X+20) = 600
3X + 3X + 60 = 600
6X + 60 = 600
6X = 540
X = 90
So one train is traveling 90 miles per hour and the other train is traveling X+20 or 110 miles per hour.