The two trains are separating at the rate of 480 mi/(4 h) = 120 mi/h. Then their average speed is (120 mi/h)/2 = 60 mi/h. The faster train will have this speed plus half the difference. The slower train will have this speed less half the difference.
The fastest train is traveling 65 mph.
The slowest train is traveling 55 mph.
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If you want an equation, you can call the speed of the slowest train "s". Then that of the fastest train is (s+10) and their sum is
s +(s +10) = 120
2s = 110
s = 110/2 = 55
s+10 = 65
