Aptitude - Problems on Trains - Discussion

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.

Another easy method to solve this problem without using the variable x. Because if we use variable x for all the problems it will b bit confusing.

Solution is in question they asked about length so the formula is.

Distance = speed * time.

Since in question they had given speed in km/hr and the remaining terms are given in seconds added to that answer too given in meters.

Henceforth, its converted to m/sec.

So for converting it is been multiplied with 5/18.
Speed = 54 * (5/18).
= 15.

Since the train and the man in same direction it should be subtracted according to the rule.

So, time = (36-20).
= 16.

Now apply it in the formula.

Distance = speed * time.
= 15 * 16.
= 240.

Answer is 240.

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?