Aptitude - Problems on Trains - Discussion

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.

@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.

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.

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.

Answer will be 30km/hr.
Because the simplification of 200/24 = 25/6.
But in the answer it's given as 25/3 that's why the answer is changed to 60 km/hr.

How come 3?

Answer - If speed of slow train is x km/h.

In question the speed of fast train is twice to slow train.

So speed of fast train = 2*(Speed of slow train).

So speed of fast train = 2*(x) = 2x.

If trains are moving in opposite direction then we ADD the Speed. And trains are in same direction then we sub speed.

Here train in same direction so we add the speed of both train.

So x + 2x = 3x.

What is formula distance?

@Aditiyabala.

Super explanation, Thank you.

If 1st train accelerated by 0.3 and 2nd 0.2 then how to solve it?

Why you are all talking always 5/18 to convert kmph into m/sec, I can't understand that logic, please explain reason behind that. Thanks in advance.