Aptitude - Problems on Trains - Discussion

Total distance to be covered to cross the bridge = Bridge Length+Train Length.

Speed = Distance/Time.
45*5/18 = X+130/30.
X: Length of Bridge.
130: Length of Train.
30: Time Taken to cross the bridge.
X = 245.

I think there is any relation b/w train length and bridge length.

So we are adding the length of the train and bridge and also the train moves opposite to bridge.

Please help me it is correct or not.

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.

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

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.

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.

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

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

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.

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.