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Two trains 200 metres and 175 metres long are running on parallel lines.
They take 7 1 seconds.when running in opposite directions 2
to pass each other. Find their speeds in km per hour.and 37 1 seconds when running in the same direction 2
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- 118 kmph ; 75 kmph
- 108 kmph ; 72 kmph
- 120 kmph ; 75 kmph
- 125 kmph ; 80 kmph
Correct Option: B
Let the speed of the train be x metre per sec. and y metre per sec. respectively.
Sum of the length of the trains = 200 + 175 = 375 metres
Case : I
When the trains are moving in opposite directions
Relative speed = (x + y) m per sec.
In this case the time taken by the trains to cross each other
| = | sec. | |
| x + y |
| ∴ | = | ||
| x + y | 2 |
⇒ x + y = 50 ...(i)
Case : II
When the trains are moving in the same direction.
Relative speed = (x – y) m per sec.
In this case, the time taken by the trains to cross each other
| = | sec. | |
| x − y |
| ∴ | = | ||
| x − y | 2 |
⇒ x – y = 10 ...(ii)
Now, x + y = 50
| x – y = 10 |
| _________ |
| 2x = 60 |
⇒ x = 30
Putting this value in equation (i),
we have
y = 50 – 30 = 20
∴ Speed of trains = 30 m per sec.
| = 30 × | = 108 km per hr. | |
| 5 |
| and 20 m per sec. = 20 × | = 72 km per hr. | |
| 5 |
