Aptitude - Problems on Trains - Discussion

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.

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.