Aptitude - Problems on Trains - Discussion

19. 

A train speeds past a pole in 15 seconds and a platform 100 m long in 25 seconds. Its length is:

[A]. 50 m
[B]. 150 m
[C]. 200 m
[D]. Data inadequate

Answer: Option B

Explanation:

Let the length of the train be x metres and its speed by y m/sec.

Then, x = 15     =>     y = x .
y 15

Therefore x + 100 = x
25 15

=> 15(x + 100) = 25x

=> 15x + 1500 = 25x

=> 1500 = 10x

=> x = 150 m.


Gowtham said: (Sep 20, 2010)  
Please explain the question in detail.

Mohanraj.A said: (Sep 21, 2010)  
Assume that length is x meters.

Speed y m/sec, speed=distance/time

so, y=x/15.------------------------1.

Like the same calculate after crossing the platform

y=x+100/25--------------------2

LHS equal. So

x+100/25=x/15;
15(x+100)=25x

Then remaing sme as above.

Prabhakaran said: (Sep 24, 2010)  
I can't understand step 2 mohanraj can you explain again clearly.

Pavan said: (Oct 6, 2010)  
Why we take x/y=15 can you please explain me?

Priya said: (Nov 15, 2010)  
@pavan

Since we have declared x-distance,y-speed
According to the below given formula,

Time=Distance/speed
Here Train speed time is specified as 15 sec so,
15 =x/y

Rajesh said: (Nov 28, 2010)  
other way is select any option and solve for both case

Jelli said: (Dec 11, 2010)  
i think i will try explain simple
1.s=d/t=x(length or distance covered by train)/15
s=x/15
2.the same train to cross cross 100m long platform is
s=d/t
x/15(it's by step1) =(100+x)/25
then we can get x=150m
thank u

Santhanalakshmi. said: (Dec 27, 2010)  
sorry for late reply..to pbn..
in train pblms length=distance..
in step 2 the length of thetrain is added with the length of the platform tat it s crossin..X+100(100 is length of the platform)
than asusual.speed=dis/tim=100+x/25
25 is the time aftrcrossing the platform.. it think u got.

Nayana said: (Jan 5, 2011)  
Hi,.

Can any one explain me why they have taken 15 sec for calculating speed of train but not 25 sec?

Jijina said: (Jan 26, 2011)  
100+L/25=SPEED
L/15=SPEED
(100+SPEED*15)=25*SPEED
100=10SPEED
SPEED=10
LENGTH L= 150.

Anil said: (Jun 9, 2011)  
Nayan correctly said! why they have taken 15 sec for calculating speed of train but not 25 sec? Since distance of signal pole is included in platform, we have to take time taken by train to pass the plateform, and not pole.

Adityabala said: (Oct 16, 2011)  
Normally we need length of the train gven data has speed and length of the platform and clue is 15 sec because the train has passed the signal post in 15 sec so we have the differnce as 25-15 we get 10 sec.

When we use 25sec as base as nareen and anil said
we should use options which option will verify us anns =10 sec
we can use 25 sec also but with the help of options we can come out .or else we should follow 15 sec as the speed of the train ok.

Hope u got it.

Adityabala said: (Oct 16, 2011)  
You see they took a signal post because it has a single point kk here we can calculate the speed .

Where as in platform we have two points so we have to add the platform length also to calculate the speed .

So guys we can calulate the speed of the train with one point is very easy So they took 15 sec as the refernce for speed of the train. got it?

If you want to calculate with 25 sec as refernce u should slove by option verification .

150+100=250mts
Total time = signalpost+platform length

25 sec = 15 sec +platform length
platform length =10 sec

so , 250mts/25sec=10mts/sec
So ans is 150 mts.

Is our ans is correct or not let us verify
10mts=1 sec
total length length of plat form time is 10 sec so ,
we have 1 sec=10 mts
10 sec=?
100mts so our ans tallied
We got length of the platform.

Shro said: (Feb 16, 2012)  
(distance or length)/speed=time
in the question both length and speed of train is not given.
So length is x and speed is y.
time given is 15sec.
so x/y=15

and in the next equation we can not write x/y so we interchange it before as y=x/15.
actually it means one and same but to use in equation in right way we have to do it in this way :)

Pavan.K said: (May 30, 2012)  
Let length =x
Case 1: speed of train in 15 sec. =x/15
Case 2: since it crossed pole then the total distance = 100+x
Now speed =100+x/25

Now equating both the speeds x/15 = 100+x/25 we get the x value

Madhavi said: (May 31, 2012)  
Thank you pavan.

Neha said: (Jan 8, 2013)  
Please let me know if we can take time as 25-15=10sec.
And solve it as,
Speed=100/10=10sec.

So D/T=S
It means x+100/25=10
x+100=10*25
x=250-100
x=150m.

Seema Duhan said: (Jul 8, 2013)  
Let train's length be x meters,

Then, train speed=train length/the time taken to cross the poll.

= x/15.

Train speed=total distance/the time to cross the platform,

x/15 = (x+100)/25.

x = 150.

James said: (Oct 24, 2013)  
Let Speed = x m/s.
Distance = 15x m = 15*10 = 150.

Time = 15 secs.
Using d = st,

15x + 100) = 25x.
x = 10.

Therefore - distance = 15x m = 15*10 = 150.

Keshab Karmakar said: (Apr 23, 2014)  
Let trains length is X meter.

When cross the pole the train goes in 1 sec = (100 + X) /25.
When cross the platform the train goes in 1 sec = X/25.

The equation:

(100 + X) /25 = X/25.
X=150.

So the length is 150 m.

Sk Lodhi said: (Sep 26, 2014)  
T = 15 Sec.
T+100 = 25 Sec.
--------------
T = 100/10 = 10 m/s where 10 sec. = 25-10(difference).
T = Length of the train
T = 15 Sec.*10 m/s=150 m.

Length of the train = 150 m.

Vinay Kumar said: (Jan 31, 2015)  
Speed*Time = Distance.

S*T = D.

T = 25-15 = 10.

S = D/t = x/15, x = length of train.

(x/15)*10 = 100.

x = 150m.

S Madhukar said: (May 15, 2015)  
Speed = Length/Time.....(1).

Distance = Speed*Time.....(2).

Time = Higher time - Lower time.

= 25-15 = 10.

Distance = 100 given.

100 = Speed*10.

Speed = 10 substitute in equation (1).

Length = 10*15 = 150.

Amit said: (Jun 28, 2015)  
Why 25-10?

Anishghoshroy said: (Aug 18, 2015)  
Let the speed of the train be x.

Then, the length of the train is 15x.

Therefore, 100+15x = 25x.

x = 10.

Which mean the speed of the train is 10 m/s as assumed above.

Then the length of train is 150 m.

Kiranjit Misra said: (Nov 23, 2015)  
Let speed = X; then length of train = 15 X;

Hence (15X+100)/X = 25.

Answer the length of train is 15X = 10*15 = 150.

Samarpita Guha said: (Mar 9, 2016)  
Let the length of the train= l meters.

We know speed = length/ time.
So, speed of the train = l/15 m/sec.

To cross the platform the train have to cover = length of the train + length of the platform [ this is the relative distance].

So total length = l+100.

l+100=(l/15)*25 [applying the rule:- length = speed*time.
=> l+100= 25l/15 m.
=> l+100= 5l/3 m.
=> 3l+300= 5l m.
=> 5l-3l= 300 m.
=> 2l=300 m.
=> l= 150 meters.

The length of the train is 150 meters.

Srinu said: (Mar 31, 2016)  
If a train crosses 300 metres long platform in 38 seconds while it crosses a signal pole in 18 seconds. What is the speed of the train in km/h?

Srinu said: (Mar 31, 2016)  
Please tell me the answer.

Jamir said: (May 1, 2016)  
Let,
Length(distance) of the train = x meters.
Here time = 15 sec.

We know, Speed = d/t
=> x/15 m/sec.
Then,
100 + x = x
----------------------------
25 15
25x = 1500 + 15x.
25x - 15x = 1500
10x = 1500
x = 150 meters ==>answer.

Babu Tunk said: (Sep 28, 2016)  
Simplest way to solve this.

A train speeds past a pole in 15 seconds and a platform 100 m long in 25 seconds.
Its length is:
Train speeds past a pole in 15 (distance/length unknown).
Train speeds past a platform of 100 m in 25 sec. it take 10 sec more to cross 100 m so to find out how much will it travel in 15 seconds is easy.

15 x 100/10 = 150.
Answer is 150 meters

Hope it helps you.

Seersha said: (Dec 6, 2016)  
We know, suppose speed = x & length = L.

L/15 = x.

Or L = 15 x.

Now,

(L +100) / 25 = x.

Can we substitute for L here.

(15 x + 100) / 25 = x.

15 x + 100 = 25 x.

100 = 10 x.

X = 10.

L = 15 * 10 = 150.

Shirish said: (Dec 9, 2016)  
Why are you adding the length of the platform and length of the train X+100?

In the question, the distance between pole and platform is nowhere mentioned.

Dhiru said: (Dec 19, 2016)  
Speed of train = length of platform/difference of time.
= 100/(25 - 15) = 10km/h.

Then distance=speed * time.
= 10 * 15 = 150 m.

Swap said: (May 2, 2017)  
Suppose the length of the train is X.
Therefore speed X/15,
As given 25 * X/15 = 100 + X,
25X = 1500 + 15X,
25X - 15X = 1500,
10 X = 1500,
X = 150.

Sangita said: (Jul 13, 2017)  
Why x multiply with 100 and exactly which formula is used here? Please, clearly explain this.

Santosh said: (Jul 21, 2017)  
Can we calculate the speed of the train by the length of platform and time taken by train to cross the platform?

Vaibhav Said said: (Aug 17, 2017)  
I can't unable to understand it, please explain.

Ravanan said: (Feb 28, 2018)  
Consider, train length is x now we find out the speed s=d/t here train length is d so s=x/15
x/15=(100+x)/25,
25x=(100+x)*15,
25x=1500+15x,
25x-15x=1500,
10x=1500,
x=1500/10,
x=150m.

Rosy said: (Aug 23, 2018)  
Please cam someone explains me again?

Mr.N.P. said: (Mar 26, 2019)  
Time Difference of Platform and Pole = 25-15= 10s.
Platform is 100m long.
speed of Train is = (D/T)=(100/10) =10m/s.
Length of pole(Train) = S * T = 10 * 15 = 150m.

Arjun said: (Aug 10, 2019)  
Let x be the length of train.

Therefore speed of train = x/15 m/sec.
x+100 = (x/15) m/sec * 25 sec.

3x+300 = 5x.
x = 300/2 =150 m.

Zeeshan said: (Aug 13, 2019)  
In very simple form:

First calculate the speed relative to platform S=D/T.
S= 100/(25-15).
S= 10m/s.

Now as S = D/T so D= S*T.
D = 10*15= 150m which is relative to Pole.
Or
D= 10*25= 250 which is relative to platform so subtract length of Platform D= 250- 100= 150m.

Sia Siam Changsan said: (Apr 19, 2020)  
Let x meter be the length of the train.
We have, speed = distance/time.
S=x/15 ( to pole),
Again, s=d/t,
x/15= x+100/25(to platform),
x=150m.

Sanjana said: (May 15, 2020)  
Good explanation.

Madhan said: (Jun 10, 2020)  
Thanks all.

Sahil Chabra said: (Jun 14, 2020)  
Can anyone explain the below step in detail?

(100+x) /25=x/15.

Debdiya said: (Jun 25, 2020)  
100 + x is the total length.
See, x is the length of the train which is assumed.
Length of the platform - 100 m,
So, 100 + x is the total length.
And as we know;

Distance = speed x time.
And speed = distance by time.

I think this helps you all.

Suraj said: (Sep 24, 2020)  
In 15 sec it crosses poll and in 25 sec it crosses the platform. Hence time to cover platform will be 25-15=10 secs.

Hence from poll to crossing the platform the length of the train will be equal to the platform which is 100m.

Now,
We need to calculate its speed:
Xkmh in 36sec.
100mtrs in 10 sec.
Then X=(36*100)/10 = 36 kmph.
Now the actual length of the train
360 mtrs in 36 secs.
X meter in 15 sec.
X = (360*15)/36 which will give you 150mtrs.

Gugi said: (Sep 25, 2020)  
Explain the answer, please.

Post your comments here:

Name *:

Email   : (optional)

» Your comments will be displayed only after manual approval.