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 3 of 5.
Javid Mir said:
1 decade ago
Let length of first train be 'L' therefore length of 2nd train is 'L/2'.
Now, since trains are moving in opposite direction, there relative speed is,
(48+42) km/h = (L+L/2) /12.
(48+42) x 5/18 m/sec = (3/2 L) /12.
90 x 5/18 x 12 = 3/2 L.
300 = 3/2 L => L = 300 x 2/3 => L = 200 mtrs.
Now with respect to Plateform, Let distance of Platform be 'X'.
Therefore, Total Length = 200 + X and Speed is of first train i.e. 48 Kms/hr & time = 45 secs.
48 kms/hr = 48x5/18 mtrs /sec.
Therefore, 48 x 5/8 = 200 + X/45.
(48x5x45) /18 = 200 + X => 600 = 200 + X => X = 600-200.
X = 400 mtrs.
Now, since trains are moving in opposite direction, there relative speed is,
(48+42) km/h = (L+L/2) /12.
(48+42) x 5/18 m/sec = (3/2 L) /12.
90 x 5/18 x 12 = 3/2 L.
300 = 3/2 L => L = 300 x 2/3 => L = 200 mtrs.
Now with respect to Plateform, Let distance of Platform be 'X'.
Therefore, Total Length = 200 + X and Speed is of first train i.e. 48 Kms/hr & time = 45 secs.
48 kms/hr = 48x5/18 mtrs /sec.
Therefore, 48 x 5/8 = 200 + X/45.
(48x5x45) /18 = 200 + X => 600 = 200 + X => X = 600-200.
X = 400 mtrs.
Piyush said:
1 decade ago
Correct Answer is 200m.
Two cross Platform Distance would be 400+y.
i.e. If train length is 200 than it should be 400+y.
Two cross Platform Distance would be 400+y.
i.e. If train length is 200 than it should be 400+y.
Sonu said:
1 decade ago
Can any one explain why should we consider second train length as "x/2"?
Sumit said:
10 years ago
Read the question carefully.
Anu said:
10 years ago
Why he took first train length? Why can't second?
Lakhbir said:
9 years ago
How come 5/18 and what it mean?
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.
Chidu said:
9 years ago
Here, mentioning the 2nd train is not needed it is a straight forward question.
Rupesh said:
9 years ago
Can you explain me at relative speed?
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)
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