Aptitude - Problems on Trains - Discussion

Each of the questions given below consists of a question followed by three statements. You have to study the question and the statements and decide which of the statement(s) is/are necessary to answer the question.

2. 

What is the speed of the train?

I. 

The train crosses a tree in 13 seconds.

II. 

The train crosses a platform of length 250 metres in 27 seconds.

 III. 

The train crosses another train running in the same direction in 32 seconds.

[A]. I and II only
[B]. II and III only
[C]. I and III only
[D]. Any two of the three
[E]. None of these

Answer: Option A

Explanation:

Let the speed of the train be x metres/sec.

Time taken to cross a tree = Length of the train
Speed of the train

Time taken to cross a platform = (Length of the train + Length of the Platform)
Speed of the train

I gives, 13 = l     13x.
x

II gives 27 = l + 250
x

13x + 250 = 27
x

x = 125 m/sec.
7

Thus I and II give the speed of the train.

The correct answer is (A.)


Preeti2009_Delhi@Yahoo.Com said: (Aug 2, 2011)  
x is the velocity of the train?125/7=17.85=velocity of the =x, in the second case,

While in the 1st case there is the tree thats why we will consider only the lengh of the train and actually we do not know that?

According to the 1st case speed of the train will be =l/13.

How we can say that the speed of the train is I,II?

Chiku said: (Jan 22, 2012)  
Here we assume that speed is x in both the cases and we just equate the two equation.

Jessy said: (Dec 17, 2013)  
Sorry I'm not clear with 125/7 please explain it. Thank you.

Syed said: (Jun 30, 2014)  
Assume Speed of the train as "x".

So,
Case I: length of the train = speed*time.
= 13*x.

Case II:

length of the train+length of the platform = 27.
-------------------------------------------
speed of the train

13x(from case I) + 250 = 27.
-----------------------
x

x = 125/7 m/sec.

Thus I and II give the speed of the train.

Nischal said: (Aug 23, 2014)  
@Syed I want to add here one thing, in given example we have tree which is being crossed by the train. As per your explanation width of the tree should be 0/1 meter.

Shinde said: (Mar 8, 2016)  
1. Let the speed of the train be x metres/sec.

Velocity = length/time Therefore, time = length/speed.

Length of train = speed*time

= 13*x...................................(1).

2. Time=(length of train+length of platform)/speed of train,

27 = (length+250)/x.

= (13x+250)/x (from 1).

= 17.85 m/sec.

Because of that I & II correct

Lakshmi said: (Feb 7, 2017)  
Why can't be option 3? Can anyone explain.

Shubham said: (Jun 14, 2017)  
Option 3 is not possible, as we will have more variable and only 1 equation. The Length of the first train is in a variable, length of 2nd again.

You will have to assume relative speed, which will again be a variable. We are supposed to fine the speed of train 1, which we won't be able to calculate.

Piklla Ganesh said: (Nov 27, 2019)  
Option 3 is not possible, as we will have more variables and only one equation. The length of the first is in a variable, length of 2nd again.

You will have to assume relative speed, which will again be a variable. We are supposed to find the speed of train 1, which we won't be able to calculate.

Nikitha said: (Jun 3, 2020)  
How come 125/7?

Kab said: (Nov 19, 2020)  
I think option B is correct.

Kab said: (Nov 19, 2020)  
Option B be possible because the third equation involves the relative speed.

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