Aptitude - Problems on Trains - Discussion

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.

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.

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.

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.

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.

(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 :)

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.

Every question they used the word " cross the pole" but here they mention " past the pole " (mean we need to reduce the time from the platform).
So time has been 25-15 = 10, T = 10.
S = 100m/10sec = 10m/sec.
S = D/T.
x + D = S*T,
X + 100 = 10* 25,
x+100 = 250,
x = 250-100,
x = 150.

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.

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.