Aptitude - Problems on Trains - Discussion

You're running at a speed of 5km/hr.
(1 km = 1000 meter and 1hr = 3600 seconds, which will be 1000/3600 = 10/36 = 5/18).
Note : for 1 km/hr.
Therefore 5(given km)*5/18 = 1.39.
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Meanwhile 125m long train crosses you in 10 seconds along with you in same direction.
Then!
125/10 i.e its in the form of (125)m/(10)s.
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So add those two values in the form of m/s, which is 13.89 = a.
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Now question to find speed of the train,since we found in the form of m/s, but the options provided are in the form of KM/HR.
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So to convert M/S to KM/HR :
(a*18/5)=(13.89*3.6).
Gives answer as 50 km/hr.

I solved this question in a different way:

When we solve the questions in which train passes a pole(or a standing man), in that case the distance traveled is equal to the length of the train. in this case,

1. The distance travelled would be = The distance traveled by the train assuming the man was standing+The distance travelled by the moving man in those 10 seconds.

= 0.125km+5*(10/3600).

= 0.139km.

Where,

0.125 is the length of the train in km.

5 is the value of speed of the man i.e. km/h.

10/3600 is the time given but converted in hr.

2. Now the speed.

As we know speed = distance/time.

Above calculated value divided by 10/3600 hr.

= 0.139/(10/3600).

= 0.139/0.00278.

= 50 km/hr.

Man speed 5 km/hr, So man's speed per second (5*5/18) m/s.

After 10 sec, train passes the man, so, man cover distance in 10 sec is :

5*5*10/18 = 13.89m.

So, train covered total distance in 10 sec is :

125 m+13.89 m = 138.89 m.

So, speed of train is :138.89/10 m/s = 13.889*3600/1000 Km/hr = 50 Km/hr.

Friends its a simple logic that if a men travel along the same direction of the train then the train speed is subtracted if it travel in opposite direction then train speed is added.

Hope you understand take this logic in your mind and solve any problem.

Why its divided by 15/8? need some brief explanation.

18/5 is used to change meter per second to kilometer per hour.

And I have no idea why it says -5 as it should be +5.

Please explain the subtract value of (x-5)?

Why train speed has to be subtracted with man speed?

Speed of man*time = 5*10 = 50.

Please give a clear cut definition for relative velocity.