Aptitude - Problems on Trains - Discussion

Thanks, everyone for explaining the answer.

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

Please explain to me.

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?

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.

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.

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

Train passes 36 sec.
Man standing 20 sec.
Train speed = 54 km/h.
platform length = ?

Train length = (54*5/18) * 36.
= 30/2 * 36.
= 30 * 18.
Train length = 540.

Platform length = (54*5/18) * 20.
= 30/2 * 20.
= 30 * 10.
= 300.

Platform length = 540 - 300.
= 240=> Ans.

Very helpful. Thanks all for giving an explanation.

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.

@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.