Question : A train of length 100 metres crosses another train of length 150 metres, running on a parallel track in the opposite direction, in 9 seconds. If the speed of a train having a length of 150 metres is 40 km/h, then what is the speed (in km/h) of the other train?
Option 1: 30
Option 2: 48
Option 3: 50
Option 4: 60
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Correct Answer: 60
Solution :
Given:
Length of 1st train = 100 m
Length of 2nd train = 150 m
Speed of 2nd train = 40 km/h
Formula used:
Time taken to cross the train = $\frac{\text{Sum of length of trains}}{\text{Their relative speed}}$
Calculation:
The relative speed = $\frac{100+150}{9} = \frac{250}{9}$m/s $= \frac{250 \times 18}{9 \times 5} = 100$ km/h
When two trains are running in opposite directions their relative speed is the sum of their speed.
⇒ 100 = 40 + Speed of other train
$\therefore$ Speed of other train $= 100 - 40 = 60$ km/h
Hence, the correct answer is 60 km/h.
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