Question 1157

 The diagram as below.

   500 m 100 km/h ->
   A<------>B (1st train)
          C<------------->D (2nd train)
            <- 80 km/h 1000 m 
 When they pass each other after t hrs,diagram as
                 A      B
                 <------>
   <------------->
   C             D
 After t hrs, B moves 100 t km and C moves 80 t km.
 So we have 100 t + 80 t = BC = CD + AB = 1000+ 500
 So, 180 t = 1500, t  = 25/3 hrs 
 [Or get t by (1000+ 500)/(100+80) directly.
 Answer: it takes 25/3 hrs for the trains to pass each other 
from the moment the locomotives are at the same point.
 Or get t by (1000 + 500)/ (1000+ 60) directly.