train problem help needed--have answer, but don't know why

How do I set this up?

Two trains start from stations that are 900 miles apart and travel toward each other on parallel tracks. One train averages 55 miles per hour, and the other train averages 65 miles per hour. How long will it take for the two trains to be 120 miles apart?

I know to use d=rt, but how to I figure out the 120 miles apart? If you could help me set up the problem I would appreciate it! Thanks!

PS The answer is 6.5 hours, but I do not know how to get there...Please help if you can. Thanks

Re: train problem help needed--have answer, but don't know w

HINT:
To be 120 miles apart, the 2 trains travelled 900-120 = 780 miles, right?
Combined speed is 65+55 = 120 mph, right?

Re: train problem help needed--have answer, but don't know w

Bingo! Got it now....Thank you SOOO much!!!!!!! Appreciate it!