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?
Answer & explanation
Correct answer: option 3
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