A train starts running from Delhi at 8 am and reaches Chandigarh at 11 am. While another train starts running from Chandigarh at 9 am and reach Delhi at 2 pm. At what time will they start to cross if they are running on parallel tracks?
Answer & explanation
Correct answer: option 3
Train 2 starts after 1 hour of train 1 i.e. 9 am . So, from 8 am to 9 am , only train 1 travels .
train 1 starts running from Delhi at 8 am and reaches Chandigarh at 11 am i.e. train 1 took 3 hours & train 2 starts running from Chandigarh at 9 am and reach Delhi at 2 pm i.e. train 2 took 5 hours .
Ratio of train 1 : train 2
time 3 : 5
speed 5 : 3
distance 15 : 15 ( distance = 5 × 3 = 15 )
Distance = Speed × Time
for 1 hour till 9 am only train 1 travels so distance covered by it = 5R
remaining distance = 10R
As trains moves in opposite direction . so relative speed = (5 + 3 )R = 8R
Time taken by trains to cross each other = \(\frac{10R}{8R}\)
= \(\frac{5}{4}\) = \(\frac{1}{4}\)
so train will cross each other at = 9 am + 1 hour 15 minute = 10:15 am