Aptitude - Problems on Trains - Discussion

@All

Here they mentioned the relative time of the train to the man is 10 seconds which means the train's length(i.e 125m) has just passed the moving man in 10 seconds,so we need to find the relative speed for the train to the man as mentioned below.

i.e, 125/10 =25/2 m/s as it is in m/s,we need to convert into km/hr
25/2 x 18/5 = 45 km/hr (relative speed between train and man)

Next we need to find the original speed of the train that passes the man's speed with the help of the relative speed between the train and man i.e,45 km/hr(,if the Direction of the man and train is opposite to one another, then do addition, if both man and train running in same direction subtract both )
Here man and train are in the same direction.
So subtract;
Let x be the speed of the train and we know the man's speed as 5 km/hr and the relative speed is 45 km/hr.
Since same direction subtraction,
x = 45+5
x = 50 km/hr (answer)
By checking:
Train's speed =50 km/hr ,,man's speed = 5 km/hr, same direction so do subtraction;
50 -5 = 45 to get the relative speed of both.

How it 18÷5 will become? Please explain.

Here is the solution for this question:

The length of a train is given as 125m (here it is mentioned as m/s), and the speed is given as 5km/hr (here it is mentioned as km/hr).

While converting km/hr into m/s, we are multiplying the speed by 5/18; therefore while converting m/s into km/hr, we have to take the reciprocal of 5/18, that is 18/5. The speed of the train should be calculated in km/hr; therefore, we take 18/5.

@Rahul.

Great work, Thanks for explaining.

Where was 18 got from? Please explain.

Why the speed of the train is calculated by 125 meters by 10sec?

How it 18÷5 will become? Please explain.

How answer is 50 km/hr? Please explain to me.

Please explain to sutract -5 but ans is 50.

So in the above problem let us consider that the speed of the person is 5 km\hr .

Leave the 5 as it is and then convert the km\hr into m\s
that means 1 km=1000m
and 60min=3600sec

Now when actually converted 3600\1000 that would give u the two zero gets cut and remains is 36\10 that would give u after dividing each by 2 i.e 18\5which means
(18m\5s)1.FRACTION

Now the major calculation,
Two object over here is Train and the Man consider them as A and B
where A is the Train as we have to find the speed of the train
then B is the speed of the Man is 5
so we get A-5

We had 125m the length of train time 10 seconds
already these are in meters and in seconds no need to convert them so 125m\10s u get (25m\2sec) 2.FRACTION

Now, finally get the first fraction and the second fraction together i.e (18\5)*(25\2)u get 45 as answer when u calculate
then A-5=45 ,when u move the negative -5 to this side of the equals too sign it converts to positive giving u A=45+5

i.e 50 so A=50 and A is the speed of train!!hope u'll understood!!take care!!

According to the question,

Trains length = 125m (distance).
Speed = 10sec,
Mans speed = 5km/hr.

The km the train hasn't travelled is not been given so take it as = X km/hr.
Man moves in the same direction = 5km/hr .

When two bodies are moving in the same direction then you will have to subtract it. If the person is moving on the opposite side he feels the train is moving faster hence u got to add But here it's on the same direction.

So, it's (x-5)km/hr.
When you want to convert kilometre into metres multiply with 5/18
So , (x-5) 5/18 ---> (1) i.e time

Time =distance/speed.
= 125/10 --> (2).

Therefore,
Time = distance/speed.
(X-5) 5/18 = 125/10,
(X-5)5/18 = 25/2,
(X-5) = 25/2 * 18/5 (simple cancellation).
(X-5) = 45,
X= 45+ 5,
X= 50.