Aptitude - Problems on Trains - Discussion

Here, the Length of the train = (15 x 20)m = 300 m.

I think we don't need 20 secs a train travelling at 54km/hr took.

36 secs to pass a platform means that it covered a distance equal to. Platform in 36 secs simply convert it and the answer is 540m.

Velocity of train v = 54km/h = 15m/s

We have a train passing 2 things, a man and a platform. Consider these to be two seperate instances.

i.e Use the velocity eqn individually for both cases :
1) For Platform:
We know v = s / t (v : velocity, s : distance, t : time)
v = 15m/s
s = l + x (l is length of train and x is length of platform)
t = 36s for passing platform
So we substitute and get the eqn :
15 = (l+x)/36
We want to find the platform length, i. e x.

2)For the man:
v = 15m/s
s = l (length of train)
t = 20s for passing man.

We get :
15 = l / 20.
Solving for l we get, l = 300m.

Subbing l into the platform eqn earlier we get:
15 = (300 + x)/36
540 = 300 + x (15 x 36 = 540)
x = 540-300 = 240m.
Bish Bash Bosh, there's the length of the platform : 240m.

Here already taken by speed in 15m/sec.
And train length 15 × 20 = 300 m.
Then why take by the agin 15m/sec× 36 seconds can you explain this?

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

Please explain to me.

Thanks, everyone for explaining the answer.

@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.

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.

t1 =36s
t2 = 20s
s= 54km/hr
l=??

d1 = s*t
d1 = 54*(5/18) * 36
d1 = 15m/s * 36
d1 = 540m.

d2 = 15*20.
d2 = 300m.3

300m is the distance up to the man standing and the length of the total platform would be greater than this. so,

300 + x = 540m.
x = 240m.

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.