Aptitude - Problems on Trains - Discussion

Can someone please tell me, we are already multiplying 2 with x then why we are again multiplying with 2 with 25/6? Please explain me.

Distance =speed *time,
100+100=speed*8.
200=speed *8,
Speed=200/8,
Speed =25m/s,
Converting m/s into km/h.
25*18/5=90km/h,
As we want twice speed so first find single speed.
1+2 one is the speed of first train and 2 twice a second so combing we get 3 now,
Speed =90/3=30.
Now twice speed is 60km/h.

Here (100+100)/8 = 3x.

200/8 = 3x.

25 = 3x.

So x = 25/3 m/s it is speed of slow train.

We want speed of fast train, it is 2x where x = 25/2 m/s.

So speed of fast train = 2*25/3 m/s.

So 50/3 m/s.

And we convert in km/s using 18/5.

So (50*18)/(3*5) = 900/15 = 60 km/h.

When you divide 200/24, you won't get 25/3? So I request you to explain me properly.

@Mounika.

We supposed the speed of the first train = x.

Nd speed of the faster train which is moving twice as fast the other = 2x (as we have given).

Now, given length of each train = 100m,

Time = 8sec,

Substitute these value in formula.

Time = (distance/speed).

8sec = (100+100/x+2x).

"Here we are adding the values b'coz the trains are in opposite direction with each other, which you can easily understand by reading the formula column. ".

8 = 200m/3x.

By cross multiplication.

24x sec = 200m.

x = 200/24.

x = 25/3 m/sec.

When we convert it into km/hr it becomes.

x = 30km/hr but.

Acc. To question they are asking for faster train which is 2x.

So by putting the value of x in 2x we'll get the correct answer.

2(30km/hr).

Answer = 60 km/hr.

@Navin.

In question they have mentioned to calculate the speed of faster train i.e to calculate 2X
X=25/3 is the speed of the slower train and ( " If one is moving twice as fast the other" given in question).

2 * X = 2 * 25/3 => 50/3.

@Adityabala.

Nice explanation. Thanks.

x = 25/3

So, speed of the faster train = 50/3 m/sec.

I can't understand these steps please help me.

In this, we are not considering the distance that is between them? Should it be considered too?

@Ikram.

Nice explanation.