A train starts from Delhi at 10.30 am and reaches Chandigarh at 12.30 pm . While another train starts from Chandigarh at 10.30 am and reaches Delhi at 12.30 pm . At what time will they start to cross each other if they are running on parallel tracks ?

11.30 am

Train 1 takes 2 hours to reach  Chandigarh from Delhi  and train 2  takes 2 sec to reach Delhi from Chandigarh .

Ratio of          train 1    :    train 2 

Time              2 hours   :       2 hours

 Time                  1       :         1    

Speed                 1       :         1

Distance          1 × 1     :       1 × 1 

Distance                 1     :        1

Distance = Speed × Time 

total distance = 2R 

As trains moves in opposite direction . so relative speed = (1 + 1 )R   =  2R m/sec

Time taken by trains to cross each other = \(\frac{2R}{2R}\) =  1 hour 

so trains will cross each other at 11:30 am .