Aptitude - Problems on Trains - Discussion

I have a simple method.

45 * 5/18 equal 12.5 m/s.
Then,
12.5 = x + 130/30 then,
12.5 * 30 = 375.
So,
375 - 130 = 245.
Therefore, x equal 245.

@Naveen Kumawat.

Nice description to understand the problem.

Hii @Sai.
Why did you subtract the two things that is length and speed?

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!

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.

Hi @Anjaneyulu,

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

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,

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

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?

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.