Aptitude - Problems on Trains - Discussion

Convert speed from km/hr to m/sec(km/hr=5/18 m/s).
Then,as we know DISTANCE=SPEED *TIME.
D = [45 * (5/18)] * 30 sec.

Then, D = 375m(This is the total distance)
So, we can subtract the total distance from the length of the train),
The length of the train is = 375 - 130 = 245m.

Length of train = 130m
Length of bridge = ?
1hr = 3600sec
= 3600/30
= 120.

1km = 1000m
= 45000/120
= 375m
The length of the train is less total.
375-130 = 245.

l = ?
Train length =130m
Speed = 45km/hr
Time = 30s.

30s = time(130+l(m)).
30s = (130+l)m/45km/hr.
30s = (130+l)m/45 * 5/18m/s.
30s = 130+l/(25/2)m/s.

30s = (130+l) * 2/25.
30 * 25 = (130 +l)* 2.
15*25 = 130 + l.
375 - 130 = l.
245m = l.

@Rajesh.

We take 5/18 to convert from km/h into m/s.

Length of bridge = (speed of the train * time taken to cross the bridge) - length of the train.
= ((45*5/18)*30) - 130,
=((25/2) * 30) -130,
=(750/2) - 130,
= 375m/s - 130s,
= 245m.

To find the length of the bridge, we need to calculate the distance the train travels in 30 seconds.

First, we convert the train's speed from km/hr to m/s:

Speed in m/s = (Speed in km/hr * 1000)/3600
So, the train's speed in m/s = (45 km/hr * 1000)/3600 = 12.5 m/s.

Now we can calculate the distance the train travels in 30 seconds:
Distance = Speed * Time.

Distance = 12.5 m/s * 30 s = 375 meters.

Since the train is 130 meters long, the remaining distance must be the length of the bridge:
Length of the bridge = Distance - Length of the train.

Length of the bridge = 375 m - 130 m = 245 meters.

Therefore, the length of the bridge is 245 meters.

My Question is why we should convert 45 km/hr into meters/per second which is a unit of speed instead of that we can convert 130/30 which is also equal to speed?

Anyone, explain this to me.

Solution:

45 * 5/18 = 12.5,
12.5 * 30 = 375,
375 - 130 = 245,
So, this is how we get 245.

45km In 1 hr.
45km In 60 min.
45 km In 60*60 seconds
45000m In 60*60 seconds
45000/120 In 30 seconds
375.m in 30 seconds.
375m including with train length so the train length is;
375 - 130 = 245 m ANS.

Distance travelled by Train= length of bridge + length of the train
So, x + 130m.
Speed = 45km/hr = 45 * 5/18 = 25/2 m/s.
Distance = speed * time.
25/2 * 30,
= 245m.