Aptitude - Problems on Trains - Discussion
Discussion Forum : Problems on Trains - General Questions (Q.No. 29)
29.
A train travelling at 48 kmph completely crosses another train having half its length and travelling in opposite direction at 42 kmph, in 12 seconds. It also passes a railway platform in 45 seconds. The length of the platform is
Answer: Option
Explanation:
Let the length of the first train be x metres.
| Then, the length of the second train is |  |
x |  |
metres. |
| 2 |
| Relative speed = (48 + 42) kmph = | |
90 x | 5 | |
m/sec = 25 m/sec. |
| 18 |
 |
[x + (x/2)] | = 12 or | 3x | = 300 or x = 200. |
| 25 | 2 |
Length of first train = 200 m.
Let the length of platform be y metres.
| Speed of the first train = | |
48 x | 5 | |
m/sec = | 40 | m/sec. |
| 18 | 3 |
| (200 + y) x |
3 | = 45 |
| 40 |
600 + 3y = 1800
y = 400 m.
Discussion:
48 comments Page 1 of 5.
Sunny Talukdar said:
6 years ago
@Sai.
RS=(48+42)=90kmh
Then 90*5/18=25m/s.
So, First Train + 2nd Train Total Length = 25m/s * 12second = 300M.
ATQ,
First Train = 200M.
2Nd Train = 100M.
Now,
First Train Speed in Ms = 48 * 5/18 = 40/3.
So, First train+platform Total Length = 40/3 * 45 = 600M.
So, platform length is =600M-200M = 400M.
@Reka.
We use first train Speed because it's moving towards the platform and more important For ATQ we use First train speed we can't use both the speed because 2nd train is moving opposite.
RS=(48+42)=90kmh
Then 90*5/18=25m/s.
So, First Train + 2nd Train Total Length = 25m/s * 12second = 300M.
ATQ,
First Train = 200M.
2Nd Train = 100M.
Now,
First Train Speed in Ms = 48 * 5/18 = 40/3.
So, First train+platform Total Length = 40/3 * 45 = 600M.
So, platform length is =600M-200M = 400M.
@Reka.
We use first train Speed because it's moving towards the platform and more important For ATQ we use First train speed we can't use both the speed because 2nd train is moving opposite.
(7)
Raj said:
4 years ago
Guys,
Why can't we assume length of 1st train 2L metres and length of 2nd train L metres?
Please explain me.
Why can't we assume length of 1st train 2L metres and length of 2nd train L metres?
Please explain me.
(3)
N.Rohith said:
5 years ago
We can do this problem in another way.
Length of train travelling at 48 km/h = 'X' metres;
Let speed of the train travelling at 48 km/h be 'u'.
Length of train travelling at 42 km/h = 'X/2' meters;
Let speed of the train travelling at 42 km/h be 'v'.
Time taken by both trains to cross each other = (X+X/2)/(u+v) {FROM FORMULA 8 IN IMPORTANT FORMULAS}.
u=48 km/h = 13.33 m/sec {CONVERTED TO metres/sec}
v=42 km/h = 11.66 m/sec {CONVERTED TO metres/sec}.
i.e, (X+X/2)/(13.33+11.66) = 12 sec.
i.e on solving , (3X)/(49.98) =12 sec.
X=199.2 metres.
Let platform length = 'Y' metres.
Time taken by a train of length 199.2 m to cross-platform of length Y m = time taken by train to cover (199.2+Y) metres. {FROM FORMULA 5 IN IMPORTANT FORMULAS}.
i.e, 45 sec = (199.2+Y)/(48 km/h).
i.e, 45 sec =(199.2+Y)/(13.33 m/sec).
From this Y=400.65 metres ; approx = 400 metres.
Length of train travelling at 48 km/h = 'X' metres;
Let speed of the train travelling at 48 km/h be 'u'.
Length of train travelling at 42 km/h = 'X/2' meters;
Let speed of the train travelling at 42 km/h be 'v'.
Time taken by both trains to cross each other = (X+X/2)/(u+v) {FROM FORMULA 8 IN IMPORTANT FORMULAS}.
u=48 km/h = 13.33 m/sec {CONVERTED TO metres/sec}
v=42 km/h = 11.66 m/sec {CONVERTED TO metres/sec}.
i.e, (X+X/2)/(13.33+11.66) = 12 sec.
i.e on solving , (3X)/(49.98) =12 sec.
X=199.2 metres.
Let platform length = 'Y' metres.
Time taken by a train of length 199.2 m to cross-platform of length Y m = time taken by train to cover (199.2+Y) metres. {FROM FORMULA 5 IN IMPORTANT FORMULAS}.
i.e, 45 sec = (199.2+Y)/(48 km/h).
i.e, 45 sec =(199.2+Y)/(13.33 m/sec).
From this Y=400.65 metres ; approx = 400 metres.
(2)
Naveen said:
5 years ago
Why we didn't use the length of the second train?
(1)
Suraj said:
9 years ago
@Lakhbir.
5/18 is used when converting kmph to mps.
18/5 is used when converting mps to kmph.
5/18 is used when converting kmph to mps.
18/5 is used when converting mps to kmph.
(1)
Shivani said:
7 years ago
As per knowledge, it is 200m.
Lakhbir said:
9 years ago
How come 5/18 and what it mean?
Chidu said:
9 years ago
Here, mentioning the 2nd train is not needed it is a straight forward question.
Aayush Sharma said:
9 years ago
The total distance that the trains have to travel to cross each other is "x" and not " x + x/2 ". Where "x" is the length of the longer train. This is where we are going wrong.
Then the length of the longer train would come out to be 300m and the length of the platform will come out to be 300m too.
Which adds up to 600m. And the train will take 45s exactly to cross the platform and the answer.
Then the length of the longer train would come out to be 300m and the length of the platform will come out to be 300m too.
Which adds up to 600m. And the train will take 45s exactly to cross the platform and the answer.
Rupesh said:
9 years ago
Can you explain me at relative speed?
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