Aptitude - Problems on Trains - Discussion

@All.

I am also confused about the 500, but we need to carefully read the question; they mentioned that the slower train crosses the driver, not the faster train.

Correct me, if I am wrong.

I think the right answer is 48 sec.

Yes, the right answer is 48 sec.

@All.

We use the length of the slower train because the problem is asking about the time it takes for the entire slower train to pass by the driver of the faster train. The calculation considers the time for the slower train to completely pass a single point (the driver), which depends on the train's full length.

The answer is 48sec since to cross the driver of the faster train you have to travel the length of the faster train plus the length of the slower train since the driver of the faster train will be sitting at the front of the train.

So.

Relative speed = 45 + 30 = 75kmph.
Converting to m/s = 75 * 5/18 = 125/6 m/s.
Time taken = (500 + 500) * 6/125 = 48 sec.

@Sahil
Speed = d/t
Speed = 125/6
t = d/speed.
t = 500/(125/6) = 500 * 6/125.

Simple:
Opposite direction (ADD the speed).

45 + 30 = 75 "answer asked in sec then convert it by multiplying 5/18 to 75".

Speed is 125/6 m/sec.
Now distance travelled by each train is its own distance and other train distance,
i.e. 500+500 =1000.
We have speed and distance use the time formula.
T = D/S.
= 1000 * 6/25.

Why do we take 500 instead of 1000?

Please anyone explain me.

What length we should consider if it is asked for faster train?

@Pinku.

i.e 24sec. Isn't it logical?

If faster train crosses slower train in 24 sec. Do you think the slower train is still not done crossing? Whether a faster train crosses a slower train same time is taken for the slower train to cross a faster train. Because they are crossing each other. If the faster train is done crossing the slower train. Then it means the slower train is also done crossing. These are the logical and physical reality. Especially a same length train or object moving in the opposite direction.