Aptitude - Problems on Trains - Discussion
Discussion Forum : Problems on Trains - General Questions (Q.No. 24)
24.
Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train be 120 metres, in what time (in seconds) will they cross each other travelling in opposite direction?
Answer: Option
Explanation:
| Speed of the first train = |  |
120 |  |
m/sec = 12 m/sec. |
| 10 |
| Speed of the second train = | |
120 | |
m/sec = 8 m/sec. |
| 15 |
Relative speed = (12 + 8) = 20 m/sec.
 Required time = |
 |
(120 + 120) | ![]](/_files/images/aptitude/1-sym-cbracket-h1.gif) |
sec = 12 sec. |
| 20 |
Discussion:
22 comments Page 1 of 3.
Mancy said:
1 decade ago
It's the speed between two objects in motion without regards to any fixed reference.
Lets take an example:If you are going 20 m/h and I'm doing 10 m/h and were headed directly toward each other, our relative speed is the sum of the two because we're coming at each other at 30 m/h.
If two trains are passing each other the relative speed can be found by adding the individual speeds of the two trains.
This applies if the trains are passing each other in opposite direction, but if they are moving in the same direction, the difference of the individual speeds would be the relative speed of the trains.
Hope this'll help you.
Lets take an example:If you are going 20 m/h and I'm doing 10 m/h and were headed directly toward each other, our relative speed is the sum of the two because we're coming at each other at 30 m/h.
If two trains are passing each other the relative speed can be found by adding the individual speeds of the two trains.
This applies if the trains are passing each other in opposite direction, but if they are moving in the same direction, the difference of the individual speeds would be the relative speed of the trains.
Hope this'll help you.
Balu said:
1 decade ago
Length of each train is X.
Speed of 1st train is V1 = X/10 m/s.
Speed of 2nd train is V2 = X/15 m/s.
Relative speed is = V1+V2.
Total distance they have to cover is 2X.
Speed = (distance) /time.
Distance = speed * time.
Answer. (X/10) + (X/15) = 2X * time.
time = 12sec.
As per given problem we can say time taken is independent on length of trains it's depend only speed of train only.
Speed of 1st train is V1 = X/10 m/s.
Speed of 2nd train is V2 = X/15 m/s.
Relative speed is = V1+V2.
Total distance they have to cover is 2X.
Speed = (distance) /time.
Distance = speed * time.
Answer. (X/10) + (X/15) = 2X * time.
time = 12sec.
As per given problem we can say time taken is independent on length of trains it's depend only speed of train only.
Surya said:
1 decade ago
I got confused while we add that (120+120) how its possible ?
In Problem number 22 we just take 500 in one time...
Y ? whats the reason we take 120 two times ?
Both 22,24 pblms are similar.
Give me a solution friends.
In Problem number 22 we just take 500 in one time...
Y ? whats the reason we take 120 two times ?
Both 22,24 pblms are similar.
Give me a solution friends.
Brajesh said:
1 decade ago
Find out the speed of first and second train when they passes t.post then add the speed as both train moves in opposite direction rellative speed is
12+8 = 20km/h
Time=distance/speed
t = 120+120/20 = 12 sec.
12+8 = 20km/h
Time=distance/speed
t = 120+120/20 = 12 sec.
Srijan das said:
6 years ago
The answer should be 6.
And 120m should be the length considered for the opposite direction.
When travelling in same direction, the length should be 120+120.
And 120m should be the length considered for the opposite direction.
When travelling in same direction, the length should be 120+120.
(1)
Harishankar said:
9 years ago
We will add 120 + 120 because of two different speeds of trains. If both trains having equal length and equal speed its not necessary to add lengths.
(2)
Vinayak PB said:
9 years ago
It's simple, just add both train durations i.e. 10s + 15s = 25s.
And just check mid point ie 25/2.
We get 12.5 so literally equal to 12s.
And just check mid point ie 25/2.
We get 12.5 so literally equal to 12s.
Nikhil said:
5 years ago
Speed of T1 = 120/10 = 12m/s.
Speed of T2 = 120/15 = 8m/s.
Time taken to cross each other = D/S = (120 + 120)/12 + 8 = 12 secs.
Speed of T2 = 120/15 = 8m/s.
Time taken to cross each other = D/S = (120 + 120)/12 + 8 = 12 secs.
(3)
Dinto said:
1 decade ago
The trains are coming in opposite direction. So ican understand the step relative speed = (12 + 8) = 20 m/sec.
Gautam said:
1 decade ago
If I am not wrong, The trains will travel 120 m each. And not 120+120. Correct me if am wrong.
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