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.
Syndhya said:
1 decade ago
Could anybody explain clearly. Please. I can't understand. Explain in a simple manner please.
Prabir kumar kar said:
1 decade ago
12 sec is used to cross both the train from opposite direction.but 45 sec is used to pass only the platform by the fastest train.
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.
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
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.
Raj said:
1 decade ago
Why should not we take length of second train as x and length of first train as 2x?
Prabu said:
1 decade ago
After found the length of the first train then it will come to know about the length of 2nd train because (x/2).
When we find the length of the platform. Why did we use the first train length?
When we find the length of the platform. Why did we use the first train length?
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.
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.
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.
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