Aptitude - Problems on Trains - Discussion

For every problem like this you should find relative speed to get solution.

Need of relative speed: we can desire the solution without forming the relation between two quantities either speed or time.

Time = (L1+L2)/(relative speed);

L1-Length of train-1.

L2-Length of train-2.

Hi friends. I have one doubt when the trains are moving in opposite direction we add the speed of those trains but why we have to add the length of the two trains. Please explain me.

How 5/18 came? Can you please explain me?

Km/hr convert meter/sec=5/18 multiple.

km/hr = 1000 m/3600 sec = 5/18 m/sec [1000/3600 = 5/18].

Why km is converted into meter though there all information is in kilometer?

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.

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

How to get 125/3?

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.