Aptitude - Problems on Trains - Discussion

5/18 came from? Can some one tell me please.

The answer will be same because the length of two two trains are same. Suppose the length of two trains are different from each other, then the time taken the by each trains driver to face the each other train edge or last part, will be different.

Better you draw a picture and take the value of speed and length a little, and then solve it, and you will understand. You can take two different length of the trains or both equal length of the train. And you will get understand the problem.

What If we had to find the time taken by fastest train instead of slowest train?

Why was the speeds added instead subtracting?

If the question is given with specific term as driver, man in the train. We must neglect the train and only consider the driver or a man! hence you should take only train length (specified train neglecting the length of train in which man or driver is present).

The question ask time taken to pass the driver of the faster train not the full train so we take the distance only 500.

Relative speed of both the trains should be consider because the faster one is also moving and it will also cover the some distance not only the slower one is moving. So since they are moving in opposite direction to each other so the relative speed will be (45+30) *5/18. And distance cover is 500+500 because both are moving so distance of both will get added. Now time taken can be calculated.

Why the distance is 500 ?

d = s/t.

(500+500) = distance = 1000.
Speed = 45kmph+30kmph = 75kmph.

Then convert it in to metre = 75*5/18 = 20.833333.
t = d/s = 1000/20.83333 = 48sec.

Then 2x(two train) = 48sec ; x = 48/2 => x = 24sec for each train.

See we can't add the lengths of two train because here there is no concept of crossing the train. When the driver of slower train would reach the end of faster train the both train will be symmetric. Means at the same time the driver of faster train would be at the end of slower train.

We would have added the lengths if asked about the crossing of guards in the last coaches.