Aptitude - Problems on Trains - Discussion

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.

@Harish.

Why do we need the length of train?"
=> when the question says " A train passes a station platform in 36 seconds" it means the time between when an engine of the train has crossed the starting point of platform till trailer had passed the end point of the platform.

If you actually analyse this, we find that train had already passed platforms total length(x) when its engine crossed end point of platform and it is still going on till its trailer crosses end point of platform(i.e. train went 300m extra which is its own length)
Trains pass length = x+300;
Trains pass time= 36 sec.

Therefore we substitute (distance) = (speed) * (time).
(x+300)= (54 * 5/18) * 36.
=> x =240m.

Another easy method to solve this problem without using the variable x. Because if we use variable x for all the problems it will b bit confusing.

Solution is in question they asked about length so the formula is.

Distance = speed * time.

Since in question they had given speed in km/hr and the remaining terms are given in seconds added to that answer too given in meters.

Henceforth, its converted to m/sec.

So for converting it is been multiplied with 5/18.
Speed = 54 * (5/18).
= 15.

Since the train and the man in same direction it should be subtracted according to the rule.

So, time = (36-20).
= 16.

Now apply it in the formula.

Distance = speed * time.
= 15 * 16.
= 240.

Answer is 240.

As we know Distance = Speed x Time.

Let's find the length of the train --> to find the distance of a train we have to multiply the speed of the train by the time taken by the train to cross a man or pole. so it comes like 15 x 20 = 300 meters.

So, the length of the train is 300 meters

Now when we multiply the train speed with the time taken to cross the platform..... we get the whole length of the train and also the platform length. LIKE --> 15 x 36 = 540 meters .

Now, this length is the whole length of the platform and the train.

To find the platform length subtract the train length that we found out with the whole length
300 - 540 = 240.

240 is the length of the platform.

There are a platform and train is passing through that platform in 36 seconds and there is a man ON THE PLATFORM, the train passes it in 20 sec. You cannot add both the times because man is standing on the platform. If he were standing outside the platform timings could have been added.

So, Trains speed is 15 m/s and it is a crossing platform in 36 seconds and men is included in it coz he is standing on the platform.
So length of train + length of platform will be = 15 multiply 36 = 540.

Now the length of the platform (you have to subtract the length of the train from total length) = 15 multiply 20 = 300.
So, 540 - 300 = 240.

@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.

@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.

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.

If the time taken by train to cross the man is 20 sec then u people cal length of train as
distance = speed * time
= 15 m/s * 20 sec
= 300m
similarly it is also given that train crosses the platform in 30sec then why not the length of the train is
distance = speed * time
= 15 m/s * 36 sec
since both man an platform are stationary not moving . and the train is crossing the platform and man in same direction
explain me some one plz ..... i very poor in aptitude .. i cant understand the logic

Friends, this method also can apply.

We know Time is taken by a train to cross a Standing man = (Length of the train) / (Speed of the train).

Thus, 20 = Length of the train/ (54 x 5/18).

So length of the train= 20 x 54 x 5/18 = 300 m.

Also, we know Time taken by a train to cross a Platform.

= (Length of the train+Length of the platform) / (Speed of the train).

I. E, 36= (300 + Length of the platform) / (54 x 5/18).

300+ So length of the platform = 36 x 54 x 5/18 = 540.

So, the length of the platform = 540 - 300 = 240 m.