Question 852388
In two hours, train A traveled 100 miles (2*50).  It had a 100-mile head start.
After three hours, train A will have traveled 3*50 or 150 miles while train B will have traveled 1*100.
After four hours, train A will have traveled 4*50 or 200 miles.  Train B will have traveled 2*100, or 200 miles.  That's where they'll meet.