Math question ?

Here's the full question:

Train A, traveling at 200 mph, leaves station A at 1 pm. Train B, traveling at 300 mph, leaves station B at 3 pm. Both stations are directly across from each other and x miles away. The trains meet at 4 pm. What is x?

At 4 pm, train A has been traveling for 3 hours.
3 hours × 200 mph = 600 miles

At that time, train B has been traveling for 1 hour.
1 hour × 300 mph = 300 miles

Answer:
x = 900 (miles)

The question states that "If the trains meet  .....". This indicates that there are two or more trains.
What is/are the other train(s) ?

  Train A, traveling at 200 mph, leaves station A at 1 pm. 
  Train B, traveling at 300 mph, leaves station B at 3 pm. 
  Both stations are directly across from each other and x miles away. 
  The trains meet at 4 pm. 
  What is x?
  x is (600 + 300) or 900 miles.

Where is Train B in your question?   You left that bit out.

Are you missing something. 

If train a leaves at 1 pm at 200 mph. At 4 pm, it would have traveled 3 hours. So 200 mph * 3 hr = 600 miles. So x us 600 mikes from train A starting point.