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.
Siddharth Kapoor said:
2 decades ago
How can 2 trains running in opposite direction cross each other!!!!
Sarika said:
2 decades ago
Two trains running in opposite directions means they come towords each other.
Dinto said:
1 decade ago
The trains are coming in opposite direction. So ican understand the step relative speed = (12 + 8) = 20 m/sec.
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.
Diya said:
1 decade ago
I am not clear with this problem. Can anyone please explain.
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.
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.
Osman said:
1 decade ago
What is relative speed?
Aayush Kuntal said:
1 decade ago
What is relative speed?
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.
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