Aptitude - Problems on Trains - Discussion

@Sudhan.

Isn't 27-17 = 10? How do you say it's 6?

Two trains are moving in opposite direction with the speed of 36 k.m. /h and 54 k.m. /h cross each other in 12 seconds. The length of the 2nd train is half of the 1st train. The 1st train crosses a platform in 1. 30 minutes. Then what is the length of the platform? Please solve this.

860m, It consists of two cases.

Very clear explanations. Thanks.

Speed=(distance/time).

It rewritten as (distance/speed)=time.
Same case:
(relative distance/relative speed)=time
We understand the only thing how to calculate relative speed and relative distance,
Consider car A travel at x km/hr and car B travels at y km/hr
Both A and B travel in same direction means
Their relative speed=subtraction of two speed (x+y)km/hr.
Relative distance= (speed1*time1)-(s2+t2).
=27x-17y.

Here 27 and 17 are time1 and time2.
X and y are speed1 and speed2.

Case2:
CarA and carB travels opposite direction.
Relative speed= x+y(addition of speeds)
Relative distance=(speed1*time1)+(s2*t2)
So we get. = 27x+17y.
Finally (relative distance/relative speed)=time,
Therefore (27x+17y)/(x+y)=23,

Maybe my solutions help for you

@Priyesh Verma.

How can be a train, which taking time than the another, is consider as faster?

In the above example, the time taken by 1 train is 23sec and the other one is 17 sec but the ratio shows that 1train is faster than second.just imagine that length of the train is 54m.

Then,
54/27::54/17~=2:3.

T1=27-23=4,T2=23-17=6.
Speed=distance/time.

"Speed" inversely propotional to "time"
(S1/S2)=(T2/T1)=6/4=3/2,
S1:S2=3:2.

Here, they mentioned 23sec and17sec but in answer, they took it as 23m and 17m why?

27 and 17 are the length of the train and the speed of these train respectively x mtr/sec and y mtr/sec.

So 27x+17x are there relative speed of the train.
(27x+17y)/(x+y)=23.
Here 23 is given time in which the Cross each other's train after solving the above eq...
We get 4x=6y.
x/y=2/3 that's solved.

Thanks for explaining the answer. It's easy to understand the solution now.