A train starts running from Delhi at 5 am and reaches Chandigarh at 9 am. While another train starts running from Chandigarh at 6 am and reach Delhi at 11 am. At what time will they start to cross if they are running on parallel tracks?

7:40 am

Train 2 starts after 1 hour of train 1 i.e. 6 am . So, from 5 am to 6 am , only train 1 travels .

 train 1 starts running from Delhi at 5 am and reaches Chandigarh at 9 am i.e. train 1 took 4 hours  &  train 2 starts running from Chandigarh at 6 am and reach Delhi at 11 am i.e. train 2 took 5 hours .

Ratio of          train 1    :    train 2 

time                  4        :       5

speed                5        :       4 

distance            20        :       20        ( distance = 5 × 4 = 20 ) 

Distance = Speed × Time 

for 1 hour till 6 am only train 1 travels so distance covered by it = 5R

remaining distance = 15R

As trains moves in opposite direction . so relative speed = (5 + 4 )R   =  9R 

Time taken by trains to cross each other = \(\frac{15R}{9R}\)

=  \(\frac{5}{3}\) =  1\(\frac{2}{3}\)

so train will cross each other at = 6 am + 1 hour 40 minute = 7:40 am