Aptitude - Problems on Trains - Discussion

@Nithin.

Distance = speed x time.
So distance = 45*5/18*30.

Distance = 375 (this is the length of both train+bridge).
So we have to subtract train length (130 m).

Distance = 245.

Why we are subtracting 130 from 375. While train runs on the bridge not off the bridge?

Length of the train = 130.
Speed of the train = 45km/hr.
Time = 30sec.

Length of bridge = x+length of train.
x+length of train = Speed of the train *time.

x+130 = 45*5/18*30.
x+130 = 375.
x = 375-130.
x = 245.

The length of the train is=130 m.
Lets assume the length of the bridge = x m.

In 30 sec the train traveled a distance of = (x+130)m.
Therefore the speed of the train = (x+130)/30 m/sec.

The speed of the train = 45 kmph.(Which is already given).
Therefore 45 kmph= (x+130)/30 m/sec.
Convert the 45 kmph into m/sec.

Then,

45* 1000/3600 = (130+x)/30.
45 * 100/ 12 = 130+x.
15 *25 = 130+x.
375 = 130+x.
==> x= 375-130= 245 m.
Therefore the the length of the bridge is 245 m.

Simple Answer to solve this problem :

Step 1> Speed of the train with respective to bridge is 45 km/hr in terms of m/s is 45*(5/18)=12.5(v-velocity) .

Step 2> let x be the length of the bridge so v=d/t where d=x+130 answer 12.5=(x+130)/30 (t-time).

245(meters) = x.

Train goes in 30 sec : 30*45*(5/18) = 375 m.
Its length = 130 m.

Therefore bridges length = (375-130)m = 245 m.

Simple method:

Speed = 45 x 5/18 m/sec = 25/2 m/sec.
Time = 30 sec.

Let the length of bridge be x metres.
Then, 130 + x /30 = 25/2.
2(130 + x) = 30*25.
2(130 + x) = 750.
260+2x = 750.
2x = 750-260.
2x = 490.
x = 490/2.
x = 245.

We know that,
Speed=45 km/hr.
=45*5/18=25/2 m/sec.

Length=speed*time.

130+x=25/2*30.
130+x=375.
x=375-130.
x=245.

25 meters per 2 sec, 1 sec = 12.5 *30sec = 375-130 = 245 very simple and very fast way to tackle the problem.

Simple logic is, we know that speed = distance/time taken.

Distance can also be represented as length.

So, speed = length/time taken.

Therefore, length = speed*time taken.

Length= (45km/hr)/30.

Convert that km/hr into m/s. This is mandatory.

Then, length = (45*5/18) *30.

Length = 375m.

But an actual length can be calculated as,

Am denoting actual length as 'x'.

Therefore, x = length-train length.

= 375-130.

= 245m.