Aptitude - Problems on Trains - Discussion

Time taken by train to pass a station platform = 36 seconds.
Time taken by train to pass a man on platform = 20 seconds.
Speed of train = 54 km/hr.
= ( 54 * 5/18,
= 3 * 5,
= 15 m/s.

length of train = 15 *20.
= 300 m.

Let the length of train be x.
Then;
300 + x /36 = 15.
300 + x = 15*36,
x= 540 -300,
x =260.

So, length of the platform is 240m.

As the man is not moving and he can be ignored as a little particle because he has not such a big length (his standing distance) as a pillar.

So if we take the time of the train passes the man, we can get the exactly length of the train.

So there, the length of the train= (15*20).

If the man would have moving, there we can't take the same length.

We are not taking the time of the platform because it has a big length which can't be ignored.

Why we have to multiple 15*20?

Just subtract the time taken by train to pass a man with the time taken by a train to pass a station i.e 36-20 = 16.

16*15(speed in meters) = 240 meters.

The train passes a station 36.
Man on platform 20.
Loss of time man 36-20=16.

Speed = distance/time.
15m/sec = L/16.
15x16 = 240 m.

Length of the train relative to the station is 36*15=540,
Length of the train relative to man@20sec is
20*15=300.

Now,
Length of the train relative to station subtract the length of man 540-300=240.

Here is the logic of why we need the train length.

Because trains enter the platform from the engine side and leave the platform from the tail side.

So it will consume more time as of the train length, therefore, we need to calculate the length of the train and subtract it from the total length we get which is 36*15 = 540 which is logical so after subtracting train length from it 540-300 = 240.

So now train when leaves the platform we subtract 300 (length of train) so it leaves the platform 300m early which engine point.

Hope this is not confusing.

(36-20)X(54X(5/18)) = 240.

Why we cannot take 15*36.

Because dist = speed x time know. 36seconds is also speed only know.

D = TxS.
As we mentioned here,
54kmph = 54x(5÷18) = 15mps.

D = TxS.
= (36-20)x15.
= 16x15.
= 240mts.