QUESTION:
One train runs from A to B at 105 miles per hour, the other runs from B to A at 85 miles per hour.  How far apart were the two trains 30 minutes prior to their crossing?.
.
.
.
.
.
.


.
.
.
.
.
.
.
SOLUTION:
They were 95 miles apart 30 minutes before the two trains crossed each other.

Since one train is traveling at 105 MPH, and the other at 85 MPH, the relative speed is 190 MPH (105 + 85).  One hour (or 60 minutes) before they crossed, they would have been 190 miles apart.  Since the question asked how far away they were 30 minutes before they crossed, then it would be half that distance, or 95 miles.