Solving for the elapsed time
distance traveled = (speed of first train + speed of second train) * elapsed time
280 miles = (95 mph + 105 mph) * elapsed time
280 miles = 200 mph * elapsed time
elapsed time = 280 miles / 200 mph
elapsed time = 1.4 hours
elapsed time = 1 hour and 24 minutes
It will take 1 hour and 24 minutes for the two trains to be 280 miles apart.