Aptitude - Problems on Trains - Discussion

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.

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.

Hi,

Could you please advise why we added x to the distance covered?

30 = 130+x/45*3.6 = 245.

Well,

36 sec --- 450 mt.

30 sec --- 375 mt.

Then, 375-135 = 275 mt.

Yes the simple method is:

Solution: Let length of the train = 130 m.

T = 30 s.

S = 45 km/hr.

= 45(5/18) m/s.
=12.5 m/s.

Distance = 12.5.

= 12.5*30 = 375.

Therefore the length of the bridge = 375-130 = 245 m.

Why we are subtracting the length of bridge?