When the second train leaves the station, the trains are already 63 miles apart. The rate of increase of their separation is (57 +63) = 120 miles per hour. The remaining 243-63 = 180 miles of separation will be achieved in ...
(180 mi)/(120 mi/h) = 1.5 h
The northbound train will have traveled 2.5 hours; the southbound train will have traveled 1.5 hours when the trains are 243 miles apart.
