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 4 of 5.
Dinesh said:
1 decade ago
Can any one explain why he took the 48 kmph in the length of the platform?
Rani said:
1 decade ago
Why they have substituted time as 12 why not 25?
The problem is related to length of platform not the length of train.
Can any one explain this?
The problem is related to length of platform not the length of train.
Can any one explain this?
Arman said:
1 decade ago
If we take 2x and x as speeds of the trains, then its changing from what it is when we take x and x/2 as two train speeds.
Lalit said:
1 decade ago
Why we are not calculating directly platform length as we have speed of train ie. 48 km and.
Sec to cross platform ie. 45 sec.
So 48*5/18 = x/45 (x is distance).
X = 600.
Sec to cross platform ie. 45 sec.
So 48*5/18 = x/45 (x is distance).
X = 600.
Rajata said:
1 decade ago
Speed of two trains in opposite direction = (48+42) = 90 km/h.
Converting it to second (90*5/18)= 25m/s.
Train passed another train = total length of the two train = 25m/s*12s= 300m As another train is half of the another train = the train length is 2*300/3 = 200m.
After the train it crossed one railway platform so we have to come to the first speed 48*5/18 = 40/3.
Then crossing time = 40/3*45 = 600 it included with the train length.
So platform length is 600-200 = 400m.
Converting it to second (90*5/18)= 25m/s.
Train passed another train = total length of the two train = 25m/s*12s= 300m As another train is half of the another train = the train length is 2*300/3 = 200m.
After the train it crossed one railway platform so we have to come to the first speed 48*5/18 = 40/3.
Then crossing time = 40/3*45 = 600 it included with the train length.
So platform length is 600-200 = 400m.
Ayesha said:
1 decade ago
@Prabha.
i.e., Because you are considering the length of first train ie. 200m. So you have to take the speed of first train only.
i.e., Because you are considering the length of first train ie. 200m. So you have to take the speed of first train only.
Raj said:
1 decade ago
Why should not we take length of second train as x and length of first train as 2x?
Chitti said:
1 decade ago
Why we taken first train speed is because the first train passes the second train and also passes the platform so we taken first train speed 48kmph.
Prabha said:
1 decade ago
Please tell me why we have to use 48km ? why not 42 km ?
after finding x=200...?? do answer it.
ie..200+x=48*(5/18)*45
after finding x=200...?? do answer it.
ie..200+x=48*(5/18)*45
Ramyajit said:
1 decade ago
@PRIYA
u r wrong.The length of the train is 200m.
To finding length of the platform,u have to solve this manner.
200+x=48*(5/18)*45
ANS:- 400m.
u r wrong.The length of the train is 200m.
To finding length of the platform,u have to solve this manner.
200+x=48*(5/18)*45
ANS:- 400m.
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