Aptitude - Problems on Trains - Discussion

Speed = 54km/hr
54*5/18 = 15m/s.

Time = 36-20 =16sec.

Length of the platform(D)=?
Distance = speed * time.
Distance = 15 * 16.
Distance = 240 m.

D = S x T.
D = (54x 5*18) x (36-20).
D = 15 x 16.
D = 240m/s.

Train and man cross each other at the same direction so time is (36s-20s) = 16s.
Distance = speed * time.
Speed = 54*5/18=15m/s.
So, distance =15 * 16 =240m.

54 * 5/18 = 15m/sec.
20 + 36sec = 56sec,
15 * 56 = 240m.

Let's take the train length for the man and station platform the same, then
D = D.
S * t = s * t.
15 * 36 = 15 * 20,
540 = 300.
540 - 300.
240.
Speed conversion 54 * 5/18 = 15.

36sec it passes both man and platform but they ask only for platform so we have to deduct man standing passes 20sec.
36-20 = 16sec it passes only for the platform.
Then find out using this formula;
Distance = speed * time.
We know speed = 54km/hr.
Time = 16sec.
D = 54*5/18 m/sec.
D = 240m.

Train cross the platform is 36sec.
A man standing (passenger for boards) is = 20 sec,
Total train running time on the platform is = 36 - 20 = 16sec.
So,
54km/hr = 54000/3600 = (540/36) = 15m/s,
platform length = 16 * 15 = 240m is the answer.

Since the options given are in meters, we must first convert given into m/s.

Speed of train =54km/hr = 54*5/18 = 15m/s.
Then find the length of the train using D = S * T eqn in the case of a man

Length of train =15 m/s *20 secs =300m.
Then while passing the platform, Total length =(train length +platform length) = 15m/s * 36 secs =540m.
Therefore platform length = 540m - 300m = 240m.

@All.

Here is the explanation:

Train speed is 54 km/h
(Sum of the seconds taken for cross=36-20 =16 sec.

If we know how many meters crossed in 1 sec we can find how long that travelled in 16 sec)
The train can cross 54 km per 1 hour.
54km =5400m (because we need ans in m)
1 hour = 60mnts and 60 sec total 3600 sec
Check the unit mentioned km/h.
We have 54000 for km and 3600 for hour.
Substitute this in km/h.
54000/3600 = 540/36 = 180/12.

= 90/6 = 15.
So the train can cross 15m per sec
So it takes 1 sec for 15m.
Then it takes 16 sec to cross.
15 * 16 = 240 m
So, the train crossed 240 m length of the platform.

Why do we choose 20sec in finding distance and not 36sec?

Please explain to me.