Aptitude - Problems on Trains - Discussion

I have solved this problem in another way.

Here am explaining it.

Distance=speed*time;

Length of train = 130m.

Speed = 45 km/hr or 45*1000/3600 m/sec.

Time = 30 sec.

Length = (45*1000/3600)*30 = 375m.

Length of bridge = 375-130 = 245m.

Hi friends, for most of these questions, just understanding the questions is enough and you don't need any formulas.

In the above problem train passes at 45 km/hr so for 1s it will go (45*5/18) = 12.5 m.

And for 30 s it will go 12. 5 * 30 = 375 m.

But we have to reduce trains length (for crossing a pole or person or bridge). So 375-130 = 245 m.

Hi !

Actually we don't know the length of the bridge so we have to assume 'x' as like length of the bridge.

The answer is in meters so we should have to convert 45 km/hr in meters. Because it is in km/hr that's why we are using 5/18.45*5/18 = 25/2.

Train travelling distance and time is 130m+x/30sec = 25/2.

After that we need to find 'x' that is 2 (130+x) = 750. If you solve this equation then you can get 'x' that's the answer.

Why we are multiplying the 60 with 5/18?

The answer is simple. We are converting the 'kilo-meters/hour' value into 'meters/second'.

Let me explain about converting km/hr into m/s in detail.

1 km = 1000 meters.
1 hr = 3600 seconds.

1 km/hr = 1000/3600 m/s

= 10/36 m/s

= 5/18 m/s.

Therefore 1 km/hr = 5/18 m/sec.

Similarly 60 km/hr = 60 x 5/18 m/sec.

You may ask me, why should I convert it from km/hr to m/sec?

Because, the time taken by the train is given in terms of seconds (9 seconds) and the 4 options are in terms of metres. But the speed of the train is given in terms of km/hr. So we have to convert the values from km/hr to m/s in order to find the solution in terms of metres/seconds.

Hope you understand better. Have a nice day guys!

Simplifying the following equation.

45*30*(5/18) = 15*25 = 375 = length of train + length of bridge.

Length of train = 375-130 = 245.

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.