could some double check this please
If train a is at the train station at 3:25 at 3:55 it will have travelled (1/2 times 100 )miles
So it is 50 miles ahead of train b
104-100 =4 difference in speeds
50/4=12.5= 12 hours 30 mins (distance apart/difference in speeds)
3:55 + 12 hours 30 mins
4:25am
But you might want to get someone else to check that

Question

could some double check this please
If train a is at the train station at 3:25  at 3:55 it will have travelled (1/2 times 100 )miles
So it is 50 miles ahead of train b
104-100 =4 difference in speeds
50/4=12.5= 12 hours 30 mins (distance apart/difference in speeds)
3:55 + 12 hours 30 mins 
4:25am
 But you might want to get someone else to check that