Aptitude - Problems on Trains - Discussion

Speed = 60 + 90.
= 150km/hr.

Length = 1.1 + 0.9.
= 2km.

Time = length/speed.
= 2/150,
= 1/75 hr (1 hr=3600s),
= 3600/75 s,
= 48sec.

Now im telling how the answer came 48 sec and doubt about 125/3.
See, the relative speed of the train in the opposite direction will be always A + B.
THEN, 90 + 60 = 150 KM/HR.

Then convert into m/s 150 * 5/18 = 125/3 clear,
Now total distance = 2 km = 2000mtr,
Required time = (2000/125/3.
= 2000/1*3/125 = 48 answer in above question this step is by passed because this is common thing which is everyone knows.

When two trains travel in parallel, then the time of train must be subtracted or added?

Length should always be added irrespective of direction and both trains are passing each other in same time interval.

T = D/S.
T = 2km/150km/hr.

Is this is right?

Please, Tell me how to solve.

Hi, @Karthik.

The relative speed between two trains is =90 Km/hr--> equation1.
But 1 Km=1000 m; and 1 hr=3600 sec.

Now these two values are substituted in equation1 because we want to convert Km/hr into m/sec because the answer was in m/sec. =(90*1000)/3600 or 90*(1000/36000).

Simplify above like in thousand two zeros are cancelled by denominator value of two zeros(3600), then we get 90*(10/36).

Then again simplify the equation by using 2 table cancel the 10 and 36, then we get 90*(5/18), then it is again simplified using 6 table cancel the 18 and 90, then we get 25*(5/3).
so, this is again simplified, and got 125/3.

How to get 125/3?

You have to add the lengths when you consider crossing each other and add speeds. For more understanding consider two trains moving at different speed and plot a graph for each sec as how much they cross each other and you will discover why the formula is:

Time for crossing = (Sum of length of two train)/(Sum of speed of two trains).

But here don't we need to consider length of longer train only because faster will cross beforehand.

Why it is explicitly mentioned "time taken for faster train to cross slower train"?

I think time taken will be same for both the trains.