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 5 of 5.
Ranjith said:
6 years ago
Well I have a little doubt in here, why can't we take the length of another train as 2x and length of this train as X so the things won't change but I can't get the ans, someone make me clear about this.
G.One said:
6 years ago
@Ranjith.
In Qn. clearly mentioned as another train length is half of the first train's length than the length of another train would be (x/2) or 0.5x rather than 2x.
In Qn. clearly mentioned as another train length is half of the first train's length than the length of another train would be (x/2) or 0.5x rather than 2x.
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)
Monika said:
5 years ago
Not getting this clearly, please explain in detail.
Naveen said:
5 years ago
Why we didn't use the length of the second train?
(1)
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)
Royal said:
7 months ago
Can anyone explain simply?
Ruko said:
6 months ago
I think 600m is the correct option.
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