Aptitude - Problems on Trains - Discussion

22. 

Two goods train each 500 m long, are running in opposite directions on parallel tracks. Their speeds are 45 km/hr and 30 km/hr respectively. Find the time taken by the slower train to pass the driver of the faster one.

[A]. 12 sec
[B]. 24 sec
[C]. 48 sec
[D]. 60 sec

Answer: Option B

Explanation:

Relative speed = = (45 + 30) km/hr
= ( 75 x 5 ( m/sec
18
= ( 125 ( m/sec.
6

We have to find the time taken by the slower train to pass the DRIVER of the faster train and not the complete train.

So, distance covered = Length of the slower train.

Therefore, Distance covered = 500 m.

Therefore Required time = ( 500 x 6 ( = 24 sec.
125


Chandana said: (Sep 22, 2010)  
Driver is starting of the train. If slower train pass the driver of the faster train automatically it pass the total train. Is it right?

Rajesh said: (Dec 4, 2010)  
Well done chandana at first I made same mistake by taking distance 1000m instead of 500.

I have got 15. Anyway, not bad.

Aarush said: (Apr 8, 2011)  
This is wrong. Try formula number 8 in formula section. We shoud use distance 1000.

Sundar said: (Apr 9, 2011)  
@All

Initially I was confused same like you. But, the given answer is correct.

We can assume the given question as:

A man running 45 km/hr opposite direction (against) to a train running 30 km/hr. Find the time taken by train to pass the man.

In the case of Man, Point, Pole.. the length will be considered as 0.

Note: In the given questions, IF it is mentioned like 'both train are running in SAME direction'.... then we can include the both train length (as Chandana said).

I hope this will help you a little bit. Have a nice day!

Vibhor said: (Apr 17, 2011)  
Ans is correct. I did mistake at first time also.

It is a good one.

Ramakhanna said: (May 10, 2011)  
For the complete train to cross each other, it takes 48 sec.

To cross only the initial phase i.e. engine of first train facing the end of the second train. and engine of second train facing the end of the first train,it takes the half of the time to cross the complete train i.e. 48sec/2=24 sec.

Vishwa said: (May 25, 2011)  
Very tricky Example...

Good one...

Manasa said: (Jun 12, 2011)  
Thank you sundar.

Sandeep said: (Aug 8, 2011)  
Its tricky one. I made mistake in first attempt. Because I didn't read the Question well and assumed that it is asking 4 the time taken to cross each other.

Good one.

We should understand the Question well before attempting it.

Sumit said: (Sep 12, 2011)  
Nothing tricky. We have to understand the concept of relative speed.

Keeping the faster train static we can consider the speed of slower train as (30+45) km/hr and as train has to pass the driver. Distance taken is 500m.

Speed=75km/hr=75*5/18m/sec.

Time taken=24sec.

Adityabala said: (Oct 17, 2011)  
Hi, Chandana asked why don't we add the length of the train. Well this is a common doubt everybody gets .

Now you think, both are travelling in opposite direction.

Eg:

Train1 --------------> Fastest train

<---------------Train2. Slowest train

When this is the case the train1 and train2 travel with their respective speed. There will be no collision rigth because they travel in opposite direction. So we take only one length of the train that too the train we respect to .ie here we take the slower train which is 500 mts length.

Hope you got it. Have a nice day!

Dhiru said: (Nov 15, 2011)  
Both trains driver meet first then time started not before meeting

E.g

Train 1.----------> Fastest train
Train 2.       <------------ Slowest train.

If they go in the way adityabala said they will never meet. So both trains driver meet when time is ZERO or they never meet.

Pinku said: (Jan 20, 2012)  
If it is 24sec, then time taken by faster train will be also 24.

But how is it possible that both are same. Not getting.

Shro said: (Feb 20, 2012)  
I agree with Pinku. If this is the way to find for slower train then what if the question is asked for faster train.

Even that will come as 24sec.

Then whats the use of asking slower and faster train in question?

OR is there any other method for it?

Manohar said: (Mar 12, 2012)  
Its a simple brain teasing question.

Dineshgct said: (May 17, 2012)  
Here they have asked only the time taken by the slower train to pass the driver of the faster train and not the length of the entire faster train.
So here we need to consider only the length of the slower train ie.500.
So,
Time=500\(75*5/18)
Which is 24sec.

Rohit Gupta said: (Sep 2, 2012)  
Its given that they are running in opposite directions and parallel to each other. And answer is given when they are running in same directions.

Rahul said: (Nov 26, 2012)  
Here length of both the trains moving opposite are same. The slower train first come across the driver of fast train. And then slow train cover its own distance to cross the driver. Now the slow train still has to cross the entire fast train.

That's why in this question we take distance as 500m i.e. length of slow train itself.

Durgesh said: (Jan 13, 2013)  
Consider the driver as a pole is best one method to understand this Question. Answer is right.

Veerappan said: (Jun 7, 2013)  
I accept @Durgesh, because he said well.

Faster train driver can be considered as an object like standing pole on station, slower train to pass a faster train driver is 500/75*5/18.

Dhiru said: (Jul 1, 2013)  
But the pole has a length of 500m so it will take additional time to cross. Thats what I think refer example 24 both seem similar guys get clear and help me to.

Suryakant Gupta said: (Aug 25, 2013)  
See we can't add the lengths of two train because here there is no concept of crossing the train. When the driver of slower train would reach the end of faster train the both train will be symmetric. Means at the same time the driver of faster train would be at the end of slower train.

We would have added the lengths if asked about the crossing of guards in the last coaches.

Msvignesh Viki said: (Jan 6, 2014)  
d = s/t.

(500+500) = distance = 1000.
Speed = 45kmph+30kmph = 75kmph.

Then convert it in to metre = 75*5/18 = 20.833333.
t = d/s = 1000/20.83333 = 48sec.

Then 2x(two train) = 48sec ; x = 48/2 => x = 24sec for each train.

Aashu said: (Feb 22, 2014)  
Why the distance is 500 ?

Manorama said: (Mar 28, 2014)  
Relative speed of both the trains should be consider because the faster one is also moving and it will also cover the some distance not only the slower one is moving. So since they are moving in opposite direction to each other so the relative speed will be (45+30) *5/18. And distance cover is 500+500 because both are moving so distance of both will get added. Now time taken can be calculated.

Ashfaq said: (Sep 6, 2014)  
The question ask time taken to pass the driver of the faster train not the full train so we take the distance only 500.

Anoo said: (Nov 18, 2014)  
If the question is given with specific term as driver, man in the train. We must neglect the train and only consider the driver or a man! hence you should take only train length (specified train neglecting the length of train in which man or driver is present).

Nana said: (Feb 7, 2015)  
Why was the speeds added instead subtracting?

Sidhant said: (Mar 9, 2015)  
What If we had to find the time taken by fastest train instead of slowest train?

Sandip said: (Apr 10, 2015)  
The answer will be same because the length of two two trains are same. Suppose the length of two trains are different from each other, then the time taken the by each trains driver to face the each other train edge or last part, will be different.

Better you draw a picture and take the value of speed and length a little, and then solve it, and you will understand. You can take two different length of the trains or both equal length of the train. And you will get understand the problem.

Baseer said: (Apr 13, 2015)  
5/18 came from? Can some one tell me please.

Swati said: (May 30, 2015)  
Nice one. We have to calculate for drive i.e. for man, not for train.

Shekhar said: (Jun 6, 2015)  
Instead of DRIVER if whole length of faster train then how to calculate?

Amar said: (Sep 17, 2015)  
In that condition, take total length both train.

i.e 1000/t = (75*5/18).

We get the answer = 48 s.

Ashu Baba said: (Dec 11, 2015)  
Conclusion is faster or slower answer will be same (examiner troll).

Alice said: (Dec 12, 2015)  
This s a good tricky question. I too made a mistake for the first time. But now I got it. Thank you for the explanation.

Ankit said: (Mar 5, 2016)  
The question here is find the time when driver meet not find the time when the train crosses each other.

Xavi said: (Apr 28, 2016)  
Yes, 24sec is the correct answer.

Because the math required us to find out only the passing time of the driver of the slower train.

Manish said: (Aug 10, 2016)  
I am not getting understand where we got 5/18.

Keerthi Srihari said: (Sep 11, 2016)  
Here there is asking only time taken by slower train to cross a faster train here assume the driver of the faster train is considered as a static object so we take only slower train distance.

Shivank said: (Oct 5, 2016)  
Here we are assuming that driver is in front of the train, but generally, drivers are not in front and if the location of the driver is mentioned question will become more tricky.

Tejaswini said: (Dec 17, 2016)  
Problem no.9 is same only the length of trains is different. There they have considered the length of both trains. Then why in this problem they have considered the length of one train?

Please tell me the reason.

Samarya Kapoor said: (Jan 16, 2017)  
@All.

Answer is incorrect. It should be 48 instead of 24. If the given answer is correct then look at question number 9 the same question with different value then q. 9 is incorrect.

Neeraj said: (Feb 3, 2017)  
@All.

This is right because of the length of the man, pole etc is negligible, here asked only driver cross.

Shyam said: (Apr 1, 2017)  
In the same question, Find the time taken by the faster train to pass the driver of the slower train one so whats the answer?

Hemanath said: (Jun 20, 2017)  
How to convert into min? Please tell me.

Pawan Yadav said: (Jul 14, 2017)  
Yes, friends.

Because the length of train add in both condition same direction or opposite direction only speed is a minus in the same direction and add opposite direction.

Nitheesh said: (Jul 23, 2017)  
What if they had asked time for a faster train?

Can anyone solve this?

Leaner said: (Aug 25, 2017)  
It should be 1000 distance because, It is clearly mention in question "500m each long".

Pallavi said: (Aug 31, 2017)  
How 5/18 coming in the problem?

Pallavi said: (Aug 31, 2017)  
The answer is simple. We are converting the 'kilo-meters/hour' value into 'meters/second'.

Let me explain about converting km/hr into m/s in detail.

1 km = 1000 meters.
1 hr = 3600 seconds.
1 km/hr = 1000/3600 m/s
= 10/36 m/s
= 5/18 m/s.

Therefore 1 km/hr = 5/18 m/sec.

Raju said: (Nov 28, 2017)  
How 125÷6 came in this problem? Please explain.

Nanda Kishor said: (Jan 20, 2018)  
@Raju.

In this problem, the relative speed is 75km/hr.
And to find the answer we have to convert the speed into m/s. So(75*5/18= 125/6m/s).
Hope that this will help you.

Ganda said: (Jan 31, 2018)  
Here answer should be 48 explain it with given example let the speed of the first train be 4m/s and second be 1m/s and length of trains be 5/5 m then you should make number line and get two seconds so we must add 5+5 10m and divides by 5m/ (1+4).

Sarang said: (May 5, 2018)  
Distance =speed * time,
500=45+30km/h*time,
500=75*5/18*time,
500=75*.277*time,
500=20.7*time,
Time=500/20.7=24s.

Shree said: (May 7, 2018)  
Thanks for explaining the answer.

Shubham said: (Jul 24, 2018)  
The correct answer is 48 sec.

Paul said: (Aug 26, 2018)  
This sum is followed by : t= (a+b)/(u+v).

Where: a is the length of the first train.
b: length of other trains.
u and v are the respective velocity.

B Kapil said: (Sep 13, 2018)  
The right answer will be 24 sec because here it is talking about the time to pass the driver, so the answer will be 24 sec, not 48sec.

Kineg said: (Oct 16, 2018)  
Can anyone explain to me how the distance came 500?

Mubarak said: (Oct 20, 2018)  
@Kineg.

Distance covered = length of the slower train.

So, it is 500m.

Jay said: (Apr 20, 2019)  
500 should be the initial distance between two trains.

Muqeet said: (Apr 22, 2019)  
Here it states to cross the driver, not cross the train.

So, please explain in detail.

Ruturaj said: (Jun 21, 2019)  
Not getting this, Please anyone explain it clearly.

Ankit Gohel said: (Aug 15, 2019)  
If speed equals then what? Please tell me.

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