A train starts running from Delhi at 7 am and reaches Chandigarh at 10 am. While another train starts running from Chandigarh at 8 am and reach Delhi at 10 am. At what time will they start to cross if they are running on parallel tracks?

8:48 am

Train 2 starts after 1 hour of train 1 i.e. 8 am . So, from 7 am to 8 am , only train 1 travels .

 train 1 starts running from Delhi at 7 am and reaches Chandigarh at 10 am i.e. train 1 took 3 hours  &  train 2 starts running from Chandigarh at 8 am and reach Delhi at 10 am i.e. train 2 took 2 hours .

Ratio of          train 1    :    train 2 

time                  3        :       2

speed                2        :       3 

distance            6        :       6        ( distance = 2 × 3 = 6 ) 

Distance = Speed × Time 

for 1 hour till 8 am only train 1 travels so distance covered by it = 2R

remaining distance = 4R

As trains moves in opposite direction . so relative speed = (2 + 3 )R   =  5R 

Time taken by trains to cross each other = \(\frac{4R}{5R}\)

=  \(\frac{4}{5}\) =  48 min

so train will cross each other at = 8 am + 48 minute = 8:48 am