Aptitude - Problems on Trains - Discussion

If the person is standing at the starting point of the platform means the platform length is 240m otherwise its length cannot be calculated. Because if the person standing between any point of platform means it will addition of part of platform and length of train.

We know that,
s = d*t.

The train has a speed of 54 km/hr.

i.e 54(5/18) = 15 m/sec.
Total distance is sum of train dist and platform distance.

Here we should consider one thing i.e a man is standing 20 sec in plat form during the train crosses the platform. Hence that will gives the length of train So the train length is,
15*20=300 m(s=d*t).

Total distance 300+x ;x is the dist of platform.

Total time 36 sec. So,
300+x/36 = 15.
Finally x = 240 meters.

Speed = (54 x 5/18) m/sec = 15 m/sec.
Length of the train = (15 x 20)m = 300 m.

Let the length of the platform be x metres.
Then x+300 = 36*15.

(Because: Time taken by a train of length l metres.
To pass a stationary object of length b metres.
Is the time taken by the train to cover (l + b)metres.)

Then x + 300 = 540.
x = 240 m.

Speed = 54*(5/18) m/sec. = 15 m/sec.
length between man and starting point of platform = (15*20) = 300 m.

x be the length between man and end point of platform and train cross the total length in 36 sec.
So,
300 + x is the total length of platform.

(300 + x)/36 = 540.

Ans = 240 m.

Sorry friends the question was asked about the total length of platform.
So the total length is,

Speed = 54*(5/18) m/sec.
Length between man and starting point of platform = ( 15*20 ) = 300 m.

x be the length between man and end point of platform and train cross the total length in 36 sec.

So,
300 + x is the total length of platform.

( 300 + x )/36 = 15.
= 240.

Total length = 300 + 240 = 540.

Instead of multiplying 15*20.

We can directly do it by method.

Firstly converting speed = 54*5/18 = 15 m.

Then as we know speed = distance/time.

Since distance = speed *time = 15(36-20) = 15*16 = 240m.

S (km/hr to m/s) => 54 * 5/18 = 15.
T = (36s - 20s) = 16s.
D = S * T = 15 * 16 = 240m.

v=d/t;. 15=d/36; d=540.

v=d/t ; 15=d/20 = d=300;

540-300 = 240.

Why length of the train will be 15*20 = 300?

Why not, length of train will be 15*36 = 540?

Please justify the reason.

Please explain why we took x+300?