Two trains leave the same station at the same time, but are headed in opposite directions. One train travels at 70 mph and the other train travels at 80 mph. How much times passes until the trains are 600 mi apart?

imagine you're on the first train. since you're moving away from second train at 70mph, and that train is moving away from you at 80mph, in total that other train for you is moving away at 150mph (as you are stationary inside the train).

if the two trains are moving apart at 150mph, then to be 600 miles apart it will take 600/150 hours, which you can calculate :)