Aptitude - Problems on Trains - Discussion

Hi,

Can anyone explain what is 5/18 here? I couldn't get this.

Train length bridge length train length.

Logically we should add train twice the train length, since crossing the bridge means train is not yet on the bridge and then it should be rolled off from bridge?

i.e , (x+2*130)/30 = 55 km/hr.

What say?

45000 meters/3600 seconds = 12.5.

12.5*30 sec = 375.

375-130 = 245.

For a quick solution:

Let the length of the bridge be x.

Speed = 45*5/18 = 75/6 m/sec.

Time = 30 sec.

Length of train 130 m.

So we know Speed = Total distance/Time.

=> Total distance = Speed*Time.

=> x+130 = 75/6*30.

=> x= 245.

Hence the length of the bridge is 245 m.

Formula given in important formulas section as:

Time taken by a train of length l metres to pass a stationery object of length b metres is the time taken by the train to cover (l+b) metres.

In the problem given train length l = 130 m.

Since bridge can be considered as a stationary object of length b.

Time for train to cross bridge is 30 s.

So by the given formula time to cross the bridge (stationary object of length b) = l+b.

30 = l+b.

=> 30 = 130+b.

b = 100.

Any body please help me why this is wrong?

Hi,

Can anyone explain what is 5/18 here? I couldn't get this.

1 hour = 60 min.
1 min = 60 sec.
1 km = 1000 m.

Km per hour to m per sec:

1 km/1 hour = 1000 m/60*60 sec.

=> 1 kmph = 1000/3600 sec = 5/18 sec.

Hi @Anjaneyulu,

5/18 is just to convert the speed km/hr into m/s.

Speed of train = 45 km/hr.

Or (45*100) m/(1*60*60) sec = (25/2) m/sec.

#----130m------>|________x meter_____________|cross in 30sec.

Train bridge.

|<--------------------130m+x---------------->|.

//# is the tail of train from where the crossing of bridge is observed.

So the total distance from # to end of the bridge is (130m+x).

And from distance formula:

Distance/Time = Speed.

i.e (130+x)/30 = 25/2.

Solving we get 245 m.

Simple Solution!

First of All change 45 KM/HR = 45000 m/s.
Travelling in 60 sec = 45000/60 = 750.

Now for 30 sec = 750/2 = 375.

Last subtract train length = 375 - 130 = 245.

I hope, this helps you!