Aptitude - Problems on Trains - Discussion

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.

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

Is this is right?

Please, Tell me how to solve.

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

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

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.

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.

2km/150km/hr * 3600.

Answer 48sec.

Here we need to consider the only length of faster trains because the question asked for the time taken by slower to pass fast train. So that what I think we need to consider relative speed but faster train length.

@Vijju.

You are right that we have to consider the faster train's length. But you have to keep concern about that both trains starts from opposite direction. So, both trains cross each others at the same time. So, when they cross each others from the opposite direction; so in that case individual length is not important. But total length of the trains is important to measure the total distance/way what they cross each others.