Aptitude - Problems on Trains - Discussion

54×5/18x(36 sec-20 sec).
= 15 × 16 = 240.

To find the length of the train why we use a 20-second time instead of 36 seconds? Anyone please explain to me.

Given,

Whole time spent on the platform as and assume it as t1 = 36 sec.
A man standing on the platform assumes it as t2 =20 sec.
We need the length of the platform.
Time (t1-t2) = 36 - 20 = 16sec.
Speed = 54 km/hr.
Convert km/hr to m/s.
1 km/hr = 1 * 5/18 m/s.
54 *5/18 = 3 * 5 = 15.
Length or distance of platform =speed * time.
=15 * 16 = 240m.

Here, the length of the train is very small compared to real life.

1) First calculate the train distance.

D = S×T.
S = 54×5/18,
S = 15m/s,
D = 15×20 = 300m.

2) Calculate the platform + train distance.
D = 36×15
= 540m
Platform distance
540 - 300 = 240m.

The speed of train = 54 x 5/18m/s,
= 15m/s.
Length of train = 15 x 20 = 300 meters ∴( distance = speed x time),
Total length = 15 x 30 = 540 (length of train x platform time taken to reach out),
The final answer is 540 - 300 meters.
= 240 meters.

Train cross the platform is 36sec.
A man standing (passenger for boards) is = 20 sec,
Total train running time on the platform is = 36 - 20 = 16sec.
So,
54km/hr = 54000/3600 = (540/36) = 15m/s,
platform length = 16 * 15 = 240m is the answer.

540 is the total distance that includes train length+ and platform length.

Here we need only platform length that's why we need train length or you can get another approach like 36-20=16sec. It's the time of starting of the train and the starting of exist platform 15 meters per sec * 16 sec = 240 meters.

How 5/18 is came sir?

@All.

Here is the explanation:

Train speed is 54 km/h.

(Sum of the seconds taken for cross = 36 - 20 =16 sec.

If we know how many meters crossed in 1 sec we can find how long that travelled in 16 sec)
The train can cross 54 km per 1 hour.
54km =5400m (because we need ans in m)
1 hour = 60mnts and 60 sec total 3600 sec.

Check the unit mentioned km/h.
We have 54000 for km and 3600 for hour.
Substitute this in km/h.
54000/3600 = 540/36 = 180/12.
= 90/6 = 15.

So, the train can cross 15m per sec;
So it takes 1 sec for 15m.
Then it takes 16 sec to cross.
15 * 16 = 240 m.
So, the train crossed 240 m length of the platform.