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?
Answer & explanation
Correct answer: option 4
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