First of all, remember if 2 people are walking away from each other at the same speed (ex 4 mph), after 1 mile, they would be 8 miles away. 4 miles south and north. So these trains go away from each other at a total of 150 mph( 67 + 83). So your equation is h = hours, 150h = 600. Divide both sides by 150 to get h = 4. 4 hours is your answer.