Given data:
The given distance is d=120 mi.
The given speed of Mark is s=30 mph.
The given speed of Mark's friend is s'=50 mph.
The expression for the distance covered by Mark when he meet with his friend at any time t is,
The expression for the distance covered by Mark's friend when he meet with Mark at any time t is,
Substitute 30t for x in the above expression.
Thus, after 1.5 hours they meet.